Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/csslib/lib/src/tree.dart

Index: mojo/public/dart/third_party/csslib/lib/src/tree.dart
diff --git a/mojo/public/dart/third_party/csslib/lib/src/tree.dart b/mojo/public/dart/third_party/csslib/lib/src/tree.dart
new file mode 100644
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@@ -0,0 +1,1425 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of csslib.visitor;
+
+/////////////////////////////////////////////////////////////////////////
+// CSS specific types:
+/////////////////////////////////////////////////////////////////////////
+
+class Identifier extends TreeNode {
+  String name;
+
+  Identifier(this.name, SourceSpan span) : super(span);
+
+  Identifier clone() => new Identifier(name, span);
+
+  visit(VisitorBase visitor) => visitor.visitIdentifier(this);
+
+  String toString() => name;
+}
+
+class Wildcard extends TreeNode {
+  Wildcard(SourceSpan span) : super(span);
+  Wildcard clone() => new Wildcard(span);
+  visit(VisitorBase visitor) => visitor.visitWildcard(this);
+
+  String get name => '*';
+}
+
+class ThisOperator extends TreeNode {
+  ThisOperator(SourceSpan span) : super(span);
+  ThisOperator clone() => new ThisOperator(span);
+  visit(VisitorBase visitor) => visitor.visitThisOperator(this);
+
+  String get name => '&';
+}
+
+class Negation extends TreeNode {
+  Negation(SourceSpan span) : super(span);
+  Negation clone() => new Negation(span);
+  visit(VisitorBase visitor) => visitor.visitNegation(this);
+
+  String get name => 'not';
+}
+
+// /*  ....   */
+class CssComment extends TreeNode {
+  final String comment;
+
+  CssComment(this.comment, SourceSpan span) : super(span);
+  CssComment clone() => new CssComment(comment, span);
+  visit(VisitorBase visitor) => visitor.visitCssComment(this);
+}
+
+// CDO/CDC (Comment Definition Open <!-- and Comment Definition Close -->).
+class CommentDefinition extends CssComment {
+  CommentDefinition(String comment, SourceSpan span) : super(comment, span);
+  CommentDefinition clone() => new CommentDefinition(comment, span);
+  visit(VisitorBase visitor) => visitor.visitCommentDefinition(this);
+}
+
+class SelectorGroup extends TreeNode {
+  final List<Selector> selectors;
+
+  SelectorGroup(this.selectors, SourceSpan span) : super(span);
+
+  SelectorGroup clone() => new SelectorGroup(selectors, span);
+
+  visit(VisitorBase visitor) => visitor.visitSelectorGroup(this);
+}
+
+class Selector extends TreeNode {
+  final List<SimpleSelectorSequence> simpleSelectorSequences;
+
+  Selector(this.simpleSelectorSequences, SourceSpan span) : super(span);
+
+  void add(SimpleSelectorSequence seq) => simpleSelectorSequences.add(seq);
+
+  int get length => simpleSelectorSequences.length;
+
+  Selector clone() {
+    var simpleSequences =
+        simpleSelectorSequences.map((ss) => ss.clone()).toList();
+
+    return new Selector(simpleSequences, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitSelector(this);
+}
+
+class SimpleSelectorSequence extends TreeNode {
+  /** +, >, ~, NONE */
+  int combinator;
+  final SimpleSelector simpleSelector;
+
+  SimpleSelectorSequence(this.simpleSelector, SourceSpan span,
+      [int combinator = TokenKind.COMBINATOR_NONE])
+      : combinator = combinator,
+        super(span);
+
+  bool get isCombinatorNone => combinator == TokenKind.COMBINATOR_NONE;
+  bool get isCombinatorPlus => combinator == TokenKind.COMBINATOR_PLUS;
+  bool get isCombinatorGreater => combinator == TokenKind.COMBINATOR_GREATER;
+  bool get isCombinatorTilde => combinator == TokenKind.COMBINATOR_TILDE;
+  bool get isCombinatorDescendant =>
+      combinator == TokenKind.COMBINATOR_DESCENDANT;
+
+  String get _combinatorToString => isCombinatorDescendant
+      ? ' '
+      : isCombinatorPlus
+          ? ' + '
+          : isCombinatorGreater ? ' > ' : isCombinatorTilde ? ' ~ ' : '';
+
+  SimpleSelectorSequence clone() =>
+      new SimpleSelectorSequence(simpleSelector, span, combinator);
+
+  visit(VisitorBase visitor) => visitor.visitSimpleSelectorSequence(this);
+
+  String toString() => simpleSelector.name;
+}
+
+/* All other selectors (element, #id, .class, attribute, pseudo, negation,
+ * namespace, *) are derived from this selector.
+ */
+abstract class SimpleSelector extends TreeNode {
+  final _name; // Wildcard, ThisOperator, Identifier, Negation, others?
+
+  SimpleSelector(this._name, SourceSpan span) : super(span);
+
+  String get name => _name.name;
+
+  bool get isWildcard => _name is Wildcard;
+
+  bool get isThis => _name is ThisOperator;
+
+  visit(VisitorBase visitor) => visitor.visitSimpleSelector(this);
+}
+
+// element name
+class ElementSelector extends SimpleSelector {
+  ElementSelector(name, SourceSpan span) : super(name, span);
+  visit(VisitorBase visitor) => visitor.visitElementSelector(this);
+
+  ElementSelector clone() => new ElementSelector(_name, span);
+
+  String toString() => name;
+}
+
+// namespace|element
+class NamespaceSelector extends SimpleSelector {
+  final _namespace; // null, Wildcard or Identifier
+
+  NamespaceSelector(this._namespace, var name, SourceSpan span)
+      : super(name, span);
+
+  String get namespace =>
+      _namespace is Wildcard ? '*' : _namespace == null ? '' : _namespace.name;
+
+  bool get isNamespaceWildcard => _namespace is Wildcard;
+
+  SimpleSelector get nameAsSimpleSelector => _name;
+
+  NamespaceSelector clone() => new NamespaceSelector(_namespace, "", span);
+
+  visit(VisitorBase visitor) => visitor.visitNamespaceSelector(this);
+
+  String toString() => "$namespace|${nameAsSimpleSelector.name}";
+}
+
+// [attr op value]
+class AttributeSelector extends SimpleSelector {
+  final int _op;
+  final _value;
+
+  AttributeSelector(Identifier name, this._op, this._value, SourceSpan span)
+      : super(name, span);
+
+  int get operatorKind => _op;
+
+  get value => _value;
+
+  String matchOperator() {
+    switch (_op) {
+      case TokenKind.EQUALS:
+        return '=';
+      case TokenKind.INCLUDES:
+        return '~=';
+      case TokenKind.DASH_MATCH:
+        return '|=';
+      case TokenKind.PREFIX_MATCH:
+        return '^=';
+      case TokenKind.SUFFIX_MATCH:
+        return '\$=';
+      case TokenKind.SUBSTRING_MATCH:
+        return '*=';
+      case TokenKind.NO_MATCH:
+        return '';
+    }
+  }
+
+  // Return the TokenKind for operator used by visitAttributeSelector.
+  String matchOperatorAsTokenString() {
+    switch (_op) {
+      case TokenKind.EQUALS:
+        return 'EQUALS';
+      case TokenKind.INCLUDES:
+        return 'INCLUDES';
+      case TokenKind.DASH_MATCH:
+        return 'DASH_MATCH';
+      case TokenKind.PREFIX_MATCH:
+        return 'PREFIX_MATCH';
+      case TokenKind.SUFFIX_MATCH:
+        return 'SUFFIX_MATCH';
+      case TokenKind.SUBSTRING_MATCH:
+        return 'SUBSTRING_MATCH';
+    }
+  }
+
+  String valueToString() {
+    if (_value != null) {
+      if (_value is Identifier) {
+        return _value.name;
+      } else {
+        return '"${_value}"';
+      }
+    } else {
+      return '';
+    }
+  }
+
+  AttributeSelector clone() => new AttributeSelector(_name, _op, _value, span);
+
+  visit(VisitorBase visitor) => visitor.visitAttributeSelector(this);
+
+  String toString() => "[$name${matchOperator()}${valueToString()}]";
+}
+
+// #id
+class IdSelector extends SimpleSelector {
+  IdSelector(Identifier name, SourceSpan span) : super(name, span);
+  IdSelector clone() => new IdSelector(_name, span);
+  visit(VisitorBase visitor) => visitor.visitIdSelector(this);
+
+  String toString() => "#$_name";
+}
+
+// .class
+class ClassSelector extends SimpleSelector {
+  ClassSelector(Identifier name, SourceSpan span) : super(name, span);
+  ClassSelector clone() => new ClassSelector(_name, span);
+  visit(VisitorBase visitor) => visitor.visitClassSelector(this);
+
+  String toString() => ".$_name";
+}
+
+// :pseudoClass
+class PseudoClassSelector extends SimpleSelector {
+  PseudoClassSelector(Identifier name, SourceSpan span) : super(name, span);
+  visit(VisitorBase visitor) => visitor.visitPseudoClassSelector(this);
+
+  PseudoClassSelector clone() => new PseudoClassSelector(_name, span);
+
+  String toString() => ":$name";
+}
+
+// ::pseudoElement
+class PseudoElementSelector extends SimpleSelector {
+  PseudoElementSelector(Identifier name, SourceSpan span) : super(name, span);
+  visit(VisitorBase visitor) => visitor.visitPseudoElementSelector(this);
+
+  PseudoElementSelector clone() => new PseudoElementSelector(_name, span);
+
+  String toString() => "::$name";
+}
+
+// :pseudoClassFunction(expression)
+class PseudoClassFunctionSelector extends PseudoClassSelector {
+  final SelectorExpression expression;
+
+  PseudoClassFunctionSelector(Identifier name, this.expression, SourceSpan span)
+      : super(name, span);
+
+  PseudoClassFunctionSelector clone() =>
+      new PseudoClassFunctionSelector(_name, expression, span);
+
+  visit(VisitorBase visitor) => visitor.visitPseudoClassFunctionSelector(this);
+}
+
+// ::pseudoElementFunction(expression)
+class PseudoElementFunctionSelector extends PseudoElementSelector {
+  final SelectorExpression expression;
+
+  PseudoElementFunctionSelector(
+      Identifier name, this.expression, SourceSpan span)
+      : super(name, span);
+
+  PseudoElementFunctionSelector clone() =>
+      new PseudoElementFunctionSelector(_name, expression, span);
+
+  visit(VisitorBase visitor) =>
+      visitor.visitPseudoElementFunctionSelector(this);
+}
+
+class SelectorExpression extends TreeNode {
+  final List<Expression> expressions;
+
+  SelectorExpression(this.expressions, SourceSpan span) : super(span);
+
+  SelectorExpression clone() {
+    return new SelectorExpression(
+        expressions.map((e) => e.clone()).toList(), span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitSelectorExpression(this);
+}
+
+// :NOT(negation_arg)
+class NegationSelector extends SimpleSelector {
+  final SimpleSelector negationArg;
+
+  NegationSelector(this.negationArg, SourceSpan span)
+      : super(new Negation(span), span);
+
+  NegationSelector clone() => new NegationSelector(negationArg, span);
+
+  visit(VisitorBase visitor) => visitor.visitNegationSelector(this);
+}
+
+class NoOp extends TreeNode {
+  NoOp() : super(null);
+
+  NoOp clone() => new NoOp();
+
+  visit(VisitorBase visitor) => visitor.visitNoOp(this);
+}
+
+class StyleSheet extends TreeNode {
+  /**
+   * Contains charset, ruleset, directives (media, page, etc.), and selectors.
+   */
+  final List<TreeNode> topLevels;
+
+  StyleSheet(this.topLevels, SourceSpan span) : super(span) {
+    for (final node in topLevels) {
+      assert(node is TopLevelProduction || node is Directive);
+    }
+  }
+
+  /** Selectors only in this tree. */
+  StyleSheet.selector(this.topLevels, SourceSpan span) : super(span);
+
+  StyleSheet clone() {
+    var clonedTopLevels = topLevels.map((e) => e.clone()).toList();
+    return new StyleSheet(clonedTopLevels, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitStyleSheet(this);
+}
+
+class TopLevelProduction extends TreeNode {
+  TopLevelProduction(SourceSpan span) : super(span);
+  TopLevelProduction clone() => new TopLevelProduction(span);
+  visit(VisitorBase visitor) => visitor.visitTopLevelProduction(this);
+}
+
+class RuleSet extends TopLevelProduction {
+  final SelectorGroup _selectorGroup;
+  final DeclarationGroup _declarationGroup;
+
+  RuleSet(this._selectorGroup, this._declarationGroup, SourceSpan span)
+      : super(span);
+
+  SelectorGroup get selectorGroup => _selectorGroup;
+  DeclarationGroup get declarationGroup => _declarationGroup;
+
+  RuleSet clone() {
+    var cloneSelectorGroup = _selectorGroup.clone();
+    var cloneDeclarationGroup = _declarationGroup.clone();
+    return new RuleSet(cloneSelectorGroup, cloneDeclarationGroup, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitRuleSet(this);
+}
+
+class Directive extends TreeNode {
+  Directive(SourceSpan span) : super(span);
+
+  bool get isBuiltIn => true; // Known CSS directive?
+  bool get isExtension => false; // SCSS extension?
+
+  Directive clone() => new Directive(span);
+  visit(VisitorBase visitor) => visitor.visitDirective(this);
+}
+
+class ImportDirective extends Directive {
+  /** import name specified. */
+  final String import;
+
+  /** Any media queries for this import. */
+  final List<MediaQuery> mediaQueries;
+
+  ImportDirective(this.import, this.mediaQueries, SourceSpan span)
+      : super(span);
+
+  ImportDirective clone() {
+    var cloneMediaQueries = [];
+    for (var mediaQuery in mediaQueries) {
+      cloneMediaQueries.add(mediaQuery.clone());
+    }
+    return new ImportDirective(import, cloneMediaQueries, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitImportDirective(this);
+}
+
+/**
+ *  MediaExpression grammar:
+ *    '(' S* media_feature S* [ ':' S* expr ]? ')' S*
+ */
+class MediaExpression extends TreeNode {
+  final bool andOperator;
+  final Identifier _mediaFeature;
+  final Expressions exprs;
+
+  MediaExpression(
+      this.andOperator, this._mediaFeature, this.exprs, SourceSpan span)
+      : super(span);
+
+  String get mediaFeature => _mediaFeature.name;
+
+  MediaExpression clone() {
+    var clonedExprs = exprs.clone();
+    return new MediaExpression(andOperator, _mediaFeature, clonedExprs, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitMediaExpression(this);
+}
+
+/**
+ * MediaQuery grammar:
+ *    : [ONLY | NOT]? S* media_type S* [ AND S* media_expression ]*
+ *    | media_expression [ AND S* media_expression ]*
+ *   media_type
+ *    : IDENT
+ *   media_expression
+ *    : '(' S* media_feature S* [ ':' S* expr ]? ')' S*
+ *   media_feature
+ *    : IDENT
+ */
+class MediaQuery extends TreeNode {
+  /** not, only or no operator. */
+  final int _mediaUnary;
+  final Identifier _mediaType;
+  final List<MediaExpression> expressions;
+
+  MediaQuery(
+      this._mediaUnary, this._mediaType, this.expressions, SourceSpan span)
+      : super(span);
+
+  bool get hasMediaType => _mediaType != null;
+  String get mediaType => _mediaType.name;
+
+  bool get hasUnary => _mediaUnary != -1;
+  String get unary =>
+      TokenKind.idToValue(TokenKind.MEDIA_OPERATORS, _mediaUnary).toUpperCase();
+
+  MediaQuery clone() {
+    var cloneExpressions = [];
+    for (var expr in expressions) {
+      cloneExpressions.add(expr.clone());
+    }
+    return new MediaQuery(_mediaUnary, _mediaType, cloneExpressions, span);
+  }
+  visit(VisitorBase visitor) => visitor.visitMediaQuery(this);
+}
+
+class MediaDirective extends Directive {
+  final List<MediaQuery> mediaQueries;
+  final List<RuleSet> rulesets;
+
+  MediaDirective(this.mediaQueries, this.rulesets, SourceSpan span)
+      : super(span);
+
+  MediaDirective clone() {
+    var cloneQueries = [];
+    for (var mediaQuery in mediaQueries) {
+      cloneQueries.add(mediaQuery.clone());
+    }
+    var cloneRulesets = [];
+    for (var ruleset in rulesets) {
+      cloneRulesets.add(ruleset.clone());
+    }
+    return new MediaDirective(cloneQueries, cloneRulesets, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitMediaDirective(this);
+}
+
+class HostDirective extends Directive {
+  final List<RuleSet> rulesets;
+
+  HostDirective(this.rulesets, SourceSpan span) : super(span);
+
+  HostDirective clone() {
+    var cloneRulesets = [];
+    for (var ruleset in rulesets) {
+      cloneRulesets.add(ruleset.clone());
+    }
+    return new HostDirective(cloneRulesets, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitHostDirective(this);
+}
+
+class PageDirective extends Directive {
+  final String _ident;
+  final String _pseudoPage;
+  final List<DeclarationGroup> _declsMargin;
+
+  PageDirective(
+      this._ident, this._pseudoPage, this._declsMargin, SourceSpan span)
+      : super(span);
+
+  PageDirective clone() {
+    var cloneDeclsMargin = [];
+    for (var declMargin in _declsMargin) {
+      cloneDeclsMargin.add(declMargin.clone());
+    }
+    return new PageDirective(_ident, _pseudoPage, cloneDeclsMargin, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitPageDirective(this);
+
+  bool get hasIdent => _ident != null && _ident.length > 0;
+  bool get hasPseudoPage => _pseudoPage != null && _pseudoPage.length > 0;
+}
+
+class CharsetDirective extends Directive {
+  final String charEncoding;
+
+  CharsetDirective(this.charEncoding, SourceSpan span) : super(span);
+  CharsetDirective clone() => new CharsetDirective(charEncoding, span);
+  visit(VisitorBase visitor) => visitor.visitCharsetDirective(this);
+}
+
+class KeyFrameDirective extends Directive {
+  /*
+   * Either @keyframe or keyframe prefixed with @-webkit-, @-moz-, @-ms-, @-o-.
+   */
+  final int _keyframeName;
+  final name;
+  final List<KeyFrameBlock> _blocks;
+
+  KeyFrameDirective(this._keyframeName, this.name, SourceSpan span)
+      : _blocks = [],
+        super(span);
+
+  add(KeyFrameBlock block) {
+    _blocks.add(block);
+  }
+
+  String get keyFrameName {
+    switch (_keyframeName) {
+      case TokenKind.DIRECTIVE_KEYFRAMES:
+      case TokenKind.DIRECTIVE_MS_KEYFRAMES:
+        return '@keyframes';
+      case TokenKind.DIRECTIVE_WEB_KIT_KEYFRAMES:
+        return '@-webkit-keyframes';
+      case TokenKind.DIRECTIVE_MOZ_KEYFRAMES:
+        return '@-moz-keyframes';
+      case TokenKind.DIRECTIVE_O_KEYFRAMES:
+        return '@-o-keyframes';
+    }
+  }
+
+  KeyFrameDirective clone() {
+    var cloneBlocks = [];
+    for (var block in _blocks) {
+      cloneBlocks.add(block.clone());
+    }
+    return new KeyFrameDirective(_keyframeName, cloneBlocks, span);
+  }
+  visit(VisitorBase visitor) => visitor.visitKeyFrameDirective(this);
+}
+
+class KeyFrameBlock extends Expression {
+  final Expressions _blockSelectors;
+  final DeclarationGroup _declarations;
+
+  KeyFrameBlock(this._blockSelectors, this._declarations, SourceSpan span)
+      : super(span);
+
+  KeyFrameBlock clone() =>
+      new KeyFrameBlock(_blockSelectors.clone(), _declarations.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitKeyFrameBlock(this);
+}
+
+class FontFaceDirective extends Directive {
+  final DeclarationGroup _declarations;
+
+  FontFaceDirective(this._declarations, SourceSpan span) : super(span);
+
+  FontFaceDirective clone() =>
+      new FontFaceDirective(_declarations.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitFontFaceDirective(this);
+}
+
+class StyletDirective extends Directive {
+  final String dartClassName;
+  final List<RuleSet> rulesets;
+
+  StyletDirective(this.dartClassName, this.rulesets, SourceSpan span)
+      : super(span);
+
+  bool get isBuiltIn => false;
+  bool get isExtension => true;
+
+  StyletDirective clone() {
+    var cloneRulesets = [];
+    for (var ruleset in rulesets) {
+      cloneRulesets.add(ruleset.clone());
+    }
+    return new StyletDirective(dartClassName, cloneRulesets, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitStyletDirective(this);
+}
+
+class NamespaceDirective extends Directive {
+  /** Namespace prefix. */
+  final String _prefix;
+
+  /** URI associated with this namespace. */
+  final String _uri;
+
+  NamespaceDirective(this._prefix, this._uri, SourceSpan span) : super(span);
+
+  NamespaceDirective clone() => new NamespaceDirective(_prefix, _uri, span);
+
+  visit(VisitorBase visitor) => visitor.visitNamespaceDirective(this);
+
+  String get prefix => _prefix.length > 0 ? '$_prefix ' : '';
+}
+
+/** To support Less syntax @name: expression */
+class VarDefinitionDirective extends Directive {
+  final VarDefinition def;
+
+  VarDefinitionDirective(this.def, SourceSpan span) : super(span);
+
+  VarDefinitionDirective clone() =>
+      new VarDefinitionDirective(def.clone(), span);
+
+  visit(VisitorBase visitor) => visitor.visitVarDefinitionDirective(this);
+}
+
+class MixinDefinition extends Directive {
+  final String name;
+  final List definedArgs;
+  final bool varArgs;
+
+  MixinDefinition(this.name, this.definedArgs, this.varArgs, SourceSpan span)
+      : super(span);
+
+  MixinDefinition clone() {
+    var cloneDefinedArgs = [];
+    for (var definedArg in definedArgs) {
+      cloneDefinedArgs.add(definedArg.clone());
+    }
+    return new MixinDefinition(name, cloneDefinedArgs, varArgs, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitMixinDefinition(this);
+}
+
+/** Support a Sass @mixin. See http://sass-lang.com for description. */
+class MixinRulesetDirective extends MixinDefinition {
+  final List<RuleSet> rulesets;
+
+  MixinRulesetDirective(String name, List<VarDefinitionDirective> args,
+      bool varArgs, this.rulesets, SourceSpan span)
+      : super(name, args, varArgs, span);
+
+  MixinRulesetDirective clone() {
+    var clonedArgs = [];
+    for (var arg in definedArgs) {
+      clonedArgs.add(arg.clone());
+    }
+    var clonedRulesets = [];
+    for (var ruleset in rulesets) {
+      clonedRulesets.add(ruleset.clone());
+    }
+    return new MixinRulesetDirective(
+        name, clonedArgs, varArgs, clonedRulesets, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitMixinRulesetDirective(this);
+}
+
+class MixinDeclarationDirective extends MixinDefinition {
+  final DeclarationGroup declarations;
+
+  MixinDeclarationDirective(String name, List<VarDefinitionDirective> args,
+      bool varArgs, this.declarations, SourceSpan span)
+      : super(name, args, varArgs, span);
+
+  MixinDeclarationDirective clone() {
+    var clonedArgs = [];
+    for (var arg in definedArgs) {
+      clonedArgs.add(arg.clone());
+    }
+    return new MixinDeclarationDirective(
+        name, clonedArgs, varArgs, declarations.clone(), span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitMixinDeclarationDirective(this);
+}
+
+/** To support consuming a SASS mixin @include. */
+class IncludeDirective extends Directive {
+  final String name;
+  final List<List<TreeNode>> args;
+
+  IncludeDirective(this.name, this.args, SourceSpan span) : super(span);
+
+  IncludeDirective clone() {
+    var cloneArgs = [];
+    for (var arg in args) {
+      for (var term in arg) {
+        cloneArgs.add(term.clone());
+      }
+    }
+    return new IncludeDirective(name, cloneArgs, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitIncludeDirective(this);
+}
+
+/** To support SASS @content. */
+class ContentDirective extends Directive {
+  ContentDirective(SourceSpan span) : super(span);
+
+  visit(VisitorBase visitor) => visitor.visitContentDirective(this);
+}
+
+class Declaration extends TreeNode {
+  final Identifier _property;
+  final Expression _expression;
+  /** Style exposed to Dart. */
+  dynamic dartStyle;
+  final bool important;
+
+  /**
+   * IE CSS hacks that can only be read by a particular IE version.
+   *   7 implies IE 7 or older property (e.g., *background: blue;)
+   *   Note:  IE 8 or older property (e.g., background: green\9;) is handled
+   *          by IE8Term in declaration expression handling.
+   *   Note:  IE 6 only property with a leading underscore is a valid IDENT
+   *          since an ident can start with underscore (e.g., _background: red;)
+   */
+  final bool isIE7;
+
+  Declaration(this._property, this._expression, this.dartStyle, SourceSpan span,
+      {important: false, ie7: false})
+      : this.important = important,
+        this.isIE7 = ie7,
+        super(span);
+
+  String get property => isIE7 ? '*${_property.name}' : _property.name;
+  Expression get expression => _expression;
+
+  bool get hasDartStyle => dartStyle != null;
+
+  Declaration clone() => new Declaration(
+      _property.clone(), _expression.clone(), dartStyle, span,
+      important: important);
+
+  visit(VisitorBase visitor) => visitor.visitDeclaration(this);
+}
+
+// TODO(terry): Consider 2 kinds of VarDefinitions static at top-level and
+//              dynamic when in a declaration.  Currently, Less syntax
+//              '@foo: expression' and 'var-foo: expression' in a declaration
+//              are statically resolved. Better solution, if @foo or var-foo
+//              are top-level are then statically resolved and var-foo in a
+//              declaration group (surrounded by a selector) would be dynamic.
+class VarDefinition extends Declaration {
+  bool badUsage = false;
+
+  VarDefinition(Identifier definedName, Expression expr, SourceSpan span)
+      : super(definedName, expr, null, span);
+
+  String get definedName => _property.name;
+
+  VarDefinition clone() => new VarDefinition(
+      _property.clone(), expression != null ? expression.clone() : null, span);
+
+  visit(VisitorBase visitor) => visitor.visitVarDefinition(this);
+}
+
+/**
+ * Node for usage of @include mixin[(args,...)] found in a declaration group
+ * instead of at a ruleset (toplevel) e.g.,
+ * div {
+ *   @include mixin1;
+ * }
+ */
+class IncludeMixinAtDeclaration extends Declaration {
+  final IncludeDirective include;
+
+  IncludeMixinAtDeclaration(this.include, SourceSpan span)
+      : super(null, null, null, span);
+
+  IncludeMixinAtDeclaration clone() =>
+      new IncludeMixinAtDeclaration(include.clone(), span);
+
+  visit(VisitorBase visitor) => visitor.visitIncludeMixinAtDeclaration(this);
+}
+
+class ExtendDeclaration extends Declaration {
+  final List<TreeNode> selectors;
+
+  ExtendDeclaration(this.selectors, SourceSpan span)
+      : super(null, null, null, span);
+
+  ExtendDeclaration clone() {
+    var newSelector = selectors.map((s) => s.clone()).toList();
+    return new ExtendDeclaration(newSelector, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitExtendDeclaration(this);
+}
+
+class DeclarationGroup extends TreeNode {
+  /** Can be either Declaration or RuleSet (if nested selector). */
+  final List declarations;
+
+  DeclarationGroup(this.declarations, SourceSpan span) : super(span);
+
+  DeclarationGroup clone() {
+    var clonedDecls = declarations.map((d) => d.clone()).toList();
+    return new DeclarationGroup(clonedDecls, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitDeclarationGroup(this);
+}
+
+class MarginGroup extends DeclarationGroup {
+  final int margin_sym; // TokenType for for @margin sym.
+
+  MarginGroup(this.margin_sym, List<Declaration> decls, SourceSpan span)
+      : super(decls, span);
+  MarginGroup clone() =>
+      new MarginGroup(margin_sym, super.clone() as dynamic, span);
+  visit(VisitorBase visitor) => visitor.visitMarginGroup(this);
+}
+
+class VarUsage extends Expression {
+  final String name;
+  final List<Expression> defaultValues;
+
+  VarUsage(this.name, this.defaultValues, SourceSpan span) : super(span);
+
+  VarUsage clone() {
+    var clonedValues = [];
+    for (var expr in defaultValues) {
+      clonedValues.add(expr.clone());
+    }
+    return new VarUsage(name, clonedValues, span);
+  }
+
+  visit(VisitorBase visitor) => visitor.visitVarUsage(this);
+}
+
+class OperatorSlash extends Expression {
+  OperatorSlash(SourceSpan span) : super(span);
+  OperatorSlash clone() => new OperatorSlash(span);
+  visit(VisitorBase visitor) => visitor.visitOperatorSlash(this);
+}
+
+class OperatorComma extends Expression {
+  OperatorComma(SourceSpan span) : super(span);
+  OperatorComma clone() => new OperatorComma(span);
+  visit(VisitorBase visitor) => visitor.visitOperatorComma(this);
+}
+
+class OperatorPlus extends Expression {
+  OperatorPlus(SourceSpan span) : super(span);
+  OperatorPlus clone() => new OperatorPlus(span);
+  visit(VisitorBase visitor) => visitor.visitOperatorPlus(this);
+}
+
+class OperatorMinus extends Expression {
+  OperatorMinus(SourceSpan span) : super(span);
+  OperatorMinus clone() => new OperatorMinus(span);
+  visit(VisitorBase visitor) => visitor.visitOperatorMinus(this);
+}
+
+class UnicodeRangeTerm extends Expression {
+  final String first;
+  final String second;
+
+  UnicodeRangeTerm(this.first, this.second, SourceSpan span) : super(span);
+
+  bool get hasSecond => second != null;
+
+  UnicodeRangeTerm clone() => new UnicodeRangeTerm(first, second, span);
+
+  visit(VisitorBase visitor) => visitor.visitUnicodeRangeTerm(this);
+}
+
+class LiteralTerm extends Expression {
+  // TODO(terry): value and text fields can be made final once all CSS resources
+  //              are copied/symlink'd in the build tool and UriVisitor in
+  //              web_ui is removed.
+  dynamic value;
+  String text;
+
+  LiteralTerm(this.value, this.text, SourceSpan span) : super(span);
+
+  LiteralTerm clone() => new LiteralTerm(value, text, span);
+
+  visit(VisitorBase visitor) => visitor.visitLiteralTerm(this);
+}
+
+class NumberTerm extends LiteralTerm {
+  NumberTerm(value, String t, SourceSpan span) : super(value, t, span);
+  NumberTerm clone() => new NumberTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitNumberTerm(this);
+}
+
+class UnitTerm extends LiteralTerm {
+  final int unit;
+
+  UnitTerm(value, String t, SourceSpan span, this.unit) : super(value, t, span);
+
+  UnitTerm clone() => new UnitTerm(value, text, span, unit);
+
+  visit(VisitorBase visitor) => visitor.visitUnitTerm(this);
+
+  String unitToString() => TokenKind.unitToString(unit);
+
+  String toString() => '$text${unitToString()}';
+}
+
+class LengthTerm extends UnitTerm {
+  LengthTerm(value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(this.unit == TokenKind.UNIT_LENGTH_PX ||
+        this.unit == TokenKind.UNIT_LENGTH_CM ||
+        this.unit == TokenKind.UNIT_LENGTH_MM ||
+        this.unit == TokenKind.UNIT_LENGTH_IN ||
+        this.unit == TokenKind.UNIT_LENGTH_PT ||
+        this.unit == TokenKind.UNIT_LENGTH_PC);
+  }
+  LengthTerm clone() => new LengthTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitLengthTerm(this);
+}
+
+class PercentageTerm extends LiteralTerm {
+  PercentageTerm(value, String t, SourceSpan span) : super(value, t, span);
+  PercentageTerm clone() => new PercentageTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitPercentageTerm(this);
+}
+
+class EmTerm extends LiteralTerm {
+  EmTerm(value, String t, SourceSpan span) : super(value, t, span);
+  EmTerm clone() => new EmTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitEmTerm(this);
+}
+
+class ExTerm extends LiteralTerm {
+  ExTerm(value, String t, SourceSpan span) : super(value, t, span);
+  ExTerm clone() => new ExTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitExTerm(this);
+}
+
+class AngleTerm extends UnitTerm {
+  AngleTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(this.unit == TokenKind.UNIT_ANGLE_DEG ||
+        this.unit == TokenKind.UNIT_ANGLE_RAD ||
+        this.unit == TokenKind.UNIT_ANGLE_GRAD ||
+        this.unit == TokenKind.UNIT_ANGLE_TURN);
+  }
+
+  AngleTerm clone() => new AngleTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitAngleTerm(this);
+}
+
+class TimeTerm extends UnitTerm {
+  TimeTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(this.unit == TokenKind.UNIT_ANGLE_DEG ||
+        this.unit == TokenKind.UNIT_TIME_MS ||
+        this.unit == TokenKind.UNIT_TIME_S);
+  }
+
+  TimeTerm clone() => new TimeTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitTimeTerm(this);
+}
+
+class FreqTerm extends UnitTerm {
+  FreqTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(unit == TokenKind.UNIT_FREQ_HZ || unit == TokenKind.UNIT_FREQ_KHZ);
+  }
+
+  FreqTerm clone() => new FreqTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitFreqTerm(this);
+}
+
+class FractionTerm extends LiteralTerm {
+  FractionTerm(var value, String t, SourceSpan span) : super(value, t, span);
+
+  FractionTerm clone() => new FractionTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitFractionTerm(this);
+}
+
+class UriTerm extends LiteralTerm {
+  UriTerm(String value, SourceSpan span) : super(value, value, span);
+
+  UriTerm clone() => new UriTerm(value, span);
+  visit(VisitorBase visitor) => visitor.visitUriTerm(this);
+}
+
+class ResolutionTerm extends UnitTerm {
+  ResolutionTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(unit == TokenKind.UNIT_RESOLUTION_DPI ||
+        unit == TokenKind.UNIT_RESOLUTION_DPCM ||
+        unit == TokenKind.UNIT_RESOLUTION_DPPX);
+  }
+
+  ResolutionTerm clone() => new ResolutionTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitResolutionTerm(this);
+}
+
+class ChTerm extends UnitTerm {
+  ChTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(unit == TokenKind.UNIT_CH);
+  }
+
+  ChTerm clone() => new ChTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitChTerm(this);
+}
+
+class RemTerm extends UnitTerm {
+  RemTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(unit == TokenKind.UNIT_REM);
+  }
+
+  RemTerm clone() => new RemTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitRemTerm(this);
+}
+
+class ViewportTerm extends UnitTerm {
+  ViewportTerm(var value, String t, SourceSpan span,
+      [int unit = TokenKind.UNIT_LENGTH_PX])
+      : super(value, t, span, unit) {
+    assert(unit == TokenKind.UNIT_VIEWPORT_VW ||
+        unit == TokenKind.UNIT_VIEWPORT_VH ||
+        unit == TokenKind.UNIT_VIEWPORT_VMIN ||
+        unit == TokenKind.UNIT_VIEWPORT_VMAX);
+  }
+
+  ViewportTerm clone() => new ViewportTerm(value, text, span, unit);
+  visit(VisitorBase visitor) => visitor.visitViewportTerm(this);
+}
+
+/** Type to signal a bad hex value for HexColorTerm.value. */
+class BAD_HEX_VALUE {}
+
+class HexColorTerm extends LiteralTerm {
+  HexColorTerm(var value, String t, SourceSpan span) : super(value, t, span);
+
+  HexColorTerm clone() => new HexColorTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitHexColorTerm(this);
+}
+
+class FunctionTerm extends LiteralTerm {
+  final Expressions _params;
+
+  FunctionTerm(var value, String t, this._params, SourceSpan span)
+      : super(value, t, span);
+
+  FunctionTerm clone() => new FunctionTerm(value, text, _params.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitFunctionTerm(this);
+}
+
+/**
+ * A "\9" was encountered at the end of the expression and before a semi-colon.
+ * This is an IE trick to ignore a property or value except by IE 8 and older
+ * browsers.
+ */
+class IE8Term extends LiteralTerm {
+  IE8Term(SourceSpan span) : super('\\9', '\\9', span);
+  IE8Term clone() => new IE8Term(span);
+  visit(VisitorBase visitor) => visitor.visitIE8Term(this);
+}
+
+class GroupTerm extends Expression {
+  final List<LiteralTerm> _terms;
+
+  GroupTerm(SourceSpan span)
+      : _terms = [],
+        super(span);
+
+  void add(LiteralTerm term) {
+    _terms.add(term);
+  }
+
+  GroupTerm clone() => new GroupTerm(span);
+  visit(VisitorBase visitor) => visitor.visitGroupTerm(this);
+}
+
+class ItemTerm extends NumberTerm {
+  ItemTerm(var value, String t, SourceSpan span) : super(value, t, span);
+
+  ItemTerm clone() => new ItemTerm(value, text, span);
+  visit(VisitorBase visitor) => visitor.visitItemTerm(this);
+}
+
+class Expressions extends Expression {
+  final List<Expression> expressions = [];
+
+  Expressions(SourceSpan span) : super(span);
+
+  void add(Expression expression) {
+    expressions.add(expression);
+  }
+
+  Expressions clone() {
+    var clonedExprs = new Expressions(span);
+    for (var expr in expressions) {
+      clonedExprs.add(expr.clone());
+    }
+    return clonedExprs;
+  }
+  visit(VisitorBase visitor) => visitor.visitExpressions(this);
+}
+
+class BinaryExpression extends Expression {
+  final Token op;
+  final Expression x;
+  final Expression y;
+
+  BinaryExpression(this.op, this.x, this.y, SourceSpan span) : super(span);
+
+  BinaryExpression clone() =>
+      new BinaryExpression(op, x.clone(), y.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitBinaryExpression(this);
+}
+
+class UnaryExpression extends Expression {
+  final Token op;
+  final Expression self;
+
+  UnaryExpression(this.op, this.self, SourceSpan span) : super(span);
+
+  UnaryExpression clone() => new UnaryExpression(op, self.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitUnaryExpression(this);
+}
+
+abstract class DartStyleExpression extends TreeNode {
+  static const int unknownType = 0;
+  static const int fontStyle = 1;
+  static const int marginStyle = 2;
+  static const int borderStyle = 3;
+  static const int paddingStyle = 4;
+  static const int heightStyle = 5;
+  static const int widthStyle = 6;
+
+  final int _styleType;
+  int priority;
+
+  DartStyleExpression(this._styleType, SourceSpan span) : super(span);
+
+  /*
+   * Merges give 2 DartStyleExpression (or derived from DartStyleExpression,
+   * e.g., FontExpression, etc.) will merge if the two expressions are of the
+   * same property name (implies same exact type e.g, FontExpression).
+   */
+  merged(DartStyleExpression newDartExpr);
+
+  bool get isUnknown => _styleType == 0 || _styleType == null;
+  bool get isFont => _styleType == fontStyle;
+  bool get isMargin => _styleType == marginStyle;
+  bool get isBorder => _styleType == borderStyle;
+  bool get isPadding => _styleType == paddingStyle;
+  bool get isHeight => _styleType == heightStyle;
+  bool get isWidth => _styleType == widthStyle;
+  bool get isBoxExpression => isMargin || isBorder || isPadding;
+
+  bool isSame(DartStyleExpression other) => this._styleType == other._styleType;
+
+  visit(VisitorBase visitor) => visitor.visitDartStyleExpression(this);
+}
+
+class FontExpression extends DartStyleExpression {
+  final Font font;
+
+  //   font-style font-variant font-weight font-size/line-height font-family
+  // TODO(terry): Only px/pt for now need to handle all possible units to
+  //              support calc expressions on units.
+  FontExpression(SourceSpan span, {dynamic size, List<String> family,
+      int weight, String style, String variant, LineHeight lineHeight})
+      : font = new Font(
+          size: size is LengthTerm ? size.value : size,
+          family: family,
+          weight: weight,
+          style: style,
+          variant: variant,
+          lineHeight: lineHeight),
+        super(DartStyleExpression.fontStyle, span);
+
+  FontExpression merged(DartStyleExpression newFontExpr) {
+    if (newFontExpr is FontExpression && this.isFont && newFontExpr.isFont) {
+      return new FontExpression.merge(this, newFontExpr);
+    }
+    return null;
+  }
+
+  /**
+   * Merge the two FontExpression and return the result.
+   */
+  factory FontExpression.merge(FontExpression x, FontExpression y) {
+    return new FontExpression._merge(x, y, y.span);
+  }
+
+  FontExpression._merge(FontExpression x, FontExpression y, SourceSpan span)
+      : font = new Font.merge(x.font, y.font),
+        super(DartStyleExpression.fontStyle, span);
+
+  FontExpression clone() => new FontExpression(span,
+      size: font.size,
+      family: font.family,
+      weight: font.weight,
+      style: font.style,
+      variant: font.variant,
+      lineHeight: font.lineHeight);
+
+  visit(VisitorBase visitor) => visitor.visitFontExpression(this);
+}
+
+abstract class BoxExpression extends DartStyleExpression {
+  final BoxEdge box;
+
+  BoxExpression(int styleType, SourceSpan span, this.box)
+      : super(styleType, span);
+
+  visit(VisitorBase visitor) => visitor.visitBoxExpression(this);
+
+  String get formattedBoxEdge {
+    if (box.top == box.left && box.top == box.bottom && box.top == box.right) {
+      return '.uniform(${box.top})';
+    } else {
+      var left = box.left == null ? 0 : box.left;
+      var top = box.top == null ? 0 : box.top;
+      var right = box.right == null ? 0 : box.right;
+      var bottom = box.bottom == null ? 0 : box.bottom;
+      return '.clockwiseFromTop($top,$right,$bottom,$left)';
+    }
+  }
+}
+
+class MarginExpression extends BoxExpression {
+  // TODO(terry): Does auto for margin need to be exposed to Dart UI framework?
+  /** Margin expression ripped apart. */
+  MarginExpression(SourceSpan span, {num top, num right, num bottom, num left})
+      : super(DartStyleExpression.marginStyle, span,
+          new BoxEdge(left, top, right, bottom));
+
+  MarginExpression.boxEdge(SourceSpan span, BoxEdge box)
+      : super(DartStyleExpression.marginStyle, span, box);
+
+  merged(DartStyleExpression newMarginExpr) {
+    if (newMarginExpr is MarginExpression &&
+        this.isMargin &&
+        newMarginExpr.isMargin) {
+      return new MarginExpression.merge(this, newMarginExpr);
+    }
+
+    return null;
+  }
+
+  /**
+   * Merge the two MarginExpressions and return the result.
+   */
+  factory MarginExpression.merge(MarginExpression x, MarginExpression y) {
+    return new MarginExpression._merge(x, y, y.span);
+  }
+
+  MarginExpression._merge(
+      MarginExpression x, MarginExpression y, SourceSpan span)
+      : super(x._styleType, span, new BoxEdge.merge(x.box, y.box));
+
+  MarginExpression clone() => new MarginExpression(span,
+      top: box.top, right: box.right, bottom: box.bottom, left: box.left);
+
+  visit(VisitorBase visitor) => visitor.visitMarginExpression(this);
+}
+
+class BorderExpression extends BoxExpression {
+  /** Border expression ripped apart. */
+  BorderExpression(SourceSpan span, {num top, num right, num bottom, num left})
+      : super(DartStyleExpression.borderStyle, span,
+          new BoxEdge(left, top, right, bottom));
+
+  BorderExpression.boxEdge(SourceSpan span, BoxEdge box)
+      : super(DartStyleExpression.borderStyle, span, box);
+
+  merged(DartStyleExpression newBorderExpr) {
+    if (newBorderExpr is BorderExpression &&
+        this.isBorder &&
+        newBorderExpr.isBorder) {
+      return new BorderExpression.merge(this, newBorderExpr);
+    }
+
+    return null;
+  }
+
+  /**
+   * Merge the two BorderExpression and return the result.
+   */
+  factory BorderExpression.merge(BorderExpression x, BorderExpression y) {
+    return new BorderExpression._merge(x, y, y.span);
+  }
+
+  BorderExpression._merge(
+      BorderExpression x, BorderExpression y, SourceSpan span)
+      : super(DartStyleExpression.borderStyle, span,
+          new BoxEdge.merge(x.box, y.box));
+
+  BorderExpression clone() => new BorderExpression(span,
+      top: box.top, right: box.right, bottom: box.bottom, left: box.left);
+
+  visit(VisitorBase visitor) => visitor.visitBorderExpression(this);
+}
+
+class HeightExpression extends DartStyleExpression {
+  final height;
+
+  HeightExpression(SourceSpan span, this.height)
+      : super(DartStyleExpression.heightStyle, span);
+
+  merged(DartStyleExpression newHeightExpr) {
+    if (newHeightExpr is DartStyleExpression &&
+        this.isHeight &&
+        newHeightExpr.isHeight) {
+      return newHeightExpr;
+    }
+
+    return null;
+  }
+
+  HeightExpression clone() => new HeightExpression(span, height);
+  visit(VisitorBase visitor) => visitor.visitHeightExpression(this);
+}
+
+class WidthExpression extends DartStyleExpression {
+  final width;
+
+  WidthExpression(SourceSpan span, this.width)
+      : super(DartStyleExpression.widthStyle, span);
+
+  merged(DartStyleExpression newWidthExpr) {
+    if (newWidthExpr is WidthExpression &&
+        this.isWidth &&
+        newWidthExpr.isWidth) {
+      return newWidthExpr;
+    }
+
+    return null;
+  }
+
+  WidthExpression clone() => new WidthExpression(span, width);
+  visit(VisitorBase visitor) => visitor.visitWidthExpression(this);
+}
+
+class PaddingExpression extends BoxExpression {
+  /** Padding expression ripped apart. */
+  PaddingExpression(SourceSpan span, {num top, num right, num bottom, num left})
+      : super(DartStyleExpression.paddingStyle, span,
+          new BoxEdge(left, top, right, bottom));
+
+  PaddingExpression.boxEdge(SourceSpan span, BoxEdge box)
+      : super(DartStyleExpression.paddingStyle, span, box);
+
+  merged(DartStyleExpression newPaddingExpr) {
+    if (newPaddingExpr is PaddingExpression &&
+        this.isPadding &&
+        newPaddingExpr.isPadding) {
+      return new PaddingExpression.merge(this, newPaddingExpr);
+    }
+
+    return null;
+  }
+
+  /**
+   * Merge the two PaddingExpression and return the result.
+   */
+  factory PaddingExpression.merge(PaddingExpression x, PaddingExpression y) {
+    return new PaddingExpression._merge(x, y, y.span);
+  }
+
+  PaddingExpression._merge(
+      PaddingExpression x, PaddingExpression y, SourceSpan span)
+      : super(DartStyleExpression.paddingStyle, span,
+          new BoxEdge.merge(x.box, y.box));
+
+  PaddingExpression clone() => new PaddingExpression(span,
+      top: box.top, right: box.right, bottom: box.bottom, left: box.left);
+  visit(VisitorBase visitor) => visitor.visitPaddingExpression(this);
+}