Added TODO.

lib/compiler/implementation/tree/nodes.dart

 // 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.
 
 abstract class Visitor<R> {
   const Visitor();
 
   abstract R visitNode(Node node);
 
   R visitBlock(Block node) => visitStatement(node);
@@ skipping 87 matching lines @@
  * A node in a syntax tree.
  *
  * The abstract part of "abstract syntax tree" is invalidated when
  * supporting tools such as code formatting. These tools need concrete
  * syntax such as parentheses and no constant folding.
  *
  * We support these tools by storing additional references back to the
  * token stream. These references are stored in fields ending with
  * "Token".
  */
-class Node implements Spannable {
+abstract class Node implements Spannable {
   final int _hashCode;
   static int _HASH_COUNTER = 0;
 
   Node() : _hashCode = ++_HASH_COUNTER;
 
   hashCode() => _hashCode;
 
   abstract accept(Visitor visitor);
 
   abstract visitChildren(Visitor visitor);
@@ skipping 20 matching lines @@
   BreakStatement asBreakStatement() => null;
   Cascade asCascade() => null;
   CascadeReceiver asCascadeReceiver() => null;
   CaseMatch asCaseMatch() => null;
   CatchBlock asCatchBlock() => null;
   ClassNode asClassNode() => null;
   Conditional asConditional() => null;
   ContinueStatement asContinueStatement() => null;
   DoWhile asDoWhile() => null;
   EmptyStatement asEmptyStatement() => null;
-  Export AsExport() => Null;
+  Export AsExport() => null;
   Expression asExpression() => null;
   ExpressionStatement asExpressionStatement() => null;
   For asFor() => null;
   ForIn asForIn() => null;
   FunctionDeclaration asFunctionDeclaration() => null;
   FunctionExpression asFunctionExpression() => null;
   Identifier asIdentifier() => null;
   If asIf() => null;
-  Import AsImport() => Null;
+  Import AsImport() => null;
   Label asLabel() => null;
   LabeledStatement asLabeledStatement() => null;
-  LibraryName AsLibraryName() => Null;
+  LibraryName AsLibraryName() => null;
   LiteralBool asLiteralBool() => null;
   LiteralDouble asLiteralDouble() => null;
   LiteralInt asLiteralInt() => null;
   LiteralList asLiteralList() => null;
   LiteralMap asLiteralMap() => null;
   LiteralMapEntry asLiteralMapEntry() => null;
   LiteralNull asLiteralNull() => null;
   LiteralString asLiteralString() => null;
   Modifiers asModifiers() => null;
   NamedArgument asNamedArgument() => null;
   NodeList asNodeList() => null;
   Operator asOperator() => null;
   ParenthesizedExpression asParenthesizedExpression() => null;
-  Part AsPart() => Null;
-  PartOf AsPartOf() => Null;
+  Part AsPart() => null;
+  PartOf AsPartOf() => null;
   Return asReturn() => null;
   ScriptTag asScriptTag() => null;
   Send asSend() => null;
   SendSet asSendSet() => null;
   Statement asStatement() => null;
   StringInterpolation asStringInterpolation() => null;
   StringInterpolationPart asStringInterpolationPart() => null;
   StringJuxtaposition asStringJuxtaposition() => null;
   StringNode asStringNode() => null;
   SwitchCase asSwitchCase() => null;
@@ skipping 42 matching lines @@
 
   bool get isInterface => beginToken.stringValue === 'interface';
 
   bool get isClass => !isInterface;
 
   Token getBeginToken() => beginToken;
 
   Token getEndToken() => endToken;
 }
 
-class Expression extends Node {
+abstract class Expression extends Node {
   Expression();
 
   Expression asExpression() => this;
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   abstract accept(Visitor visitor);
 }
 
-class Statement extends Node {
+abstract class Statement extends Node {
   Statement();
 
   Statement asStatement() => this;
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   abstract accept(Visitor visitor);
 
   bool isValidBreakTarget() => true;
 }
 
@@ skipping 410 matching lines @@
 
   Token getEndToken() {
     Token token = (body === null) ? null : body.getEndToken();
     token = (token === null) ? parameters.getEndToken() : token;
     return (token === null) ? name.getEndToken() : token;
   }
 }
 
 typedef void DecodeErrorHandler(Token token, var error);
 
-class Literal<T> extends Expression {
+abstract class Literal<T> extends Expression {
   final Token token;
   final DecodeErrorHandler handler;
 
   Literal(Token this.token, DecodeErrorHandler this.handler);
 
   abstract T get value;
 
   visitChildren(Visitor visitor) {}
 
   Token getBeginToken() => token;
@@ skipping 118 matching lines @@
   static StringQuoting getQuoting(int quote, bool raw, int quoteLength) {
     int index = quoteLength - 1;
     if (quoteLength > 2) index -= 1;
     return mapping[(raw ? 1 : 0) + index * 2 + (quote === $SQ ? 8 : 0)];
   }
 }
 
 /**
   * Superclass for classes representing string literals.
   */
-class StringNode extends Expression {
+abstract class StringNode extends Expression {
   abstract DartString get dartString;
   abstract bool get isInterpolation;
 
   StringNode asStringNode() => this;
 }
 
 class LiteralString extends StringNode {
   final Token token;
   /** Non-null on validated string literals. */
   final DartString dartString;
@@ skipping 210 matching lines @@
     var token = firstBeginToken(modifiers, type);
     if (token === null) {
       token = definitions.getBeginToken();
     }
     return token;
   }
 
   Token getEndToken() => endToken;
 }
 
-class Loop extends Statement {
+abstract class Loop extends Statement {
   abstract Expression get condition;
   final Statement body;
 
   Loop(this.body);
 
   bool isValidContinueTarget() => true;
 }
 
 class DoWhile extends Loop {
   final Token doKeyword;
@@ skipping 421 matching lines @@
         return defaultKeyword.next;
       }
       // The colon after the last expression.
       return labelsAndCases.getEndToken();
     } else {
       return statements.getEndToken();
     }
   }
 }
 
-class GotoStatement extends Statement {
+abstract class GotoStatement extends Statement {
   final Identifier target;
   final Token keywordToken;
   final Token semicolonToken;
 
   GotoStatement(this.target, this.keywordToken, this.semicolonToken);
 
   visitChildren(Visitor visitor) {
     if (target !== null) target.accept(visitor);
   }
 
@@ skipping 139 matching lines @@
     } else if (isLibrary()) {
       return new LibraryName(tag.token, argument);
     } else if (isSource()) {
       return new Part(tag.token, argument);
     } else {
       throw 'Unknown script tag ${tag.token.slowToString()}';
     }
   }
 }
 
-class LibraryTag extends Node {
+abstract class LibraryTag extends Node {
   bool get isLibraryName => false;
   bool get isImport => false;
   bool get isExport => false;
   bool get isPart => false;
   bool get isPartOf => false;
 }
 
 class LibraryName extends LibraryTag {
   final Expression name;
 
@@ skipping 97 matching lines @@
   final Expression name;
 
   final Token partKeyword;
 
   PartOf(this.partKeyword, this.name);
 
   Token get ofKeyword => partKeyword.next;
 
   bool get isPartOf => true;
 
-  Part asPartOf() => this;
+  PartOf asPartOf() => this;
 
   accept(Visitor visitor) => visitor.visitPartOf(this);
 
   visitChildren(Visitor visitor) => name.accept(visitor);
 
   Token getBeginToken() => partKeyword;
 
   Token getEndToken() => name.getEndToken().next;
 }
 
@@ skipping 171 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send !== null) return f(send.arguments.head);
 }