Move markdown library.

utils/markdown/ast.dart

-// Copyright (c) 2011, 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.
-
-/// Base class for any AST item. Roughly corresponds to Node in the DOM. Will
-/// be either an Element or Text.
-interface Node {
-  void accept(NodeVisitor visitor);
-}
-
-/// A named tag that can contain other nodes.
-class Element implements Node {
-  final String tag;
-  final List<Node> children;
-  final Map<String, String> attributes;
-
-  Element(this.tag, this.children)
-    : attributes = <String>{};
-
-  Element.empty(this.tag)
-    : children = null,
-      attributes = <String>{};
-
-  Element.tag(this.tag)
-    : children = [],
-      attributes = <String>{};
-
-  Element.text(this.tag, String text)
-    : children = [new Text(text)],
-      attributes = <String>{};
-
-  bool get isEmpty() => children == null;
-
-  void accept(NodeVisitor visitor) {
-    if (visitor.visitElementBefore(this)) {
-      for (final child in children) child.accept(visitor);
-      visitor.visitElementAfter(this);
-    }
-  }
-}
-
-/// A plain text element.
-class Text implements Node {
-  final String text;
-  Text(this.text);
-
-  void accept(NodeVisitor visitor) => visitor.visitText(this);
-}
-
-/// Visitor pattern for the AST. Renderers or other AST transformers should
-/// implement this.
-interface NodeVisitor {
-  /// Called when a Text node has been reached.
-  void visitText(Text text);
-
-  /// Called when an Element has been reached, before its children have been
-  /// visited. Return `false` to skip its children.
-  bool visitElementBefore(Element element);
-
-  /// Called when an Element has been reached, after its children have been
-  /// visited. Will not be called if [visitElementBefore] returns `false`.
-  void visitElementAfter(Element element);
-}