First pass at a markdown parser in Dart.

utils/markdown/ast.dart

Index: utils/markdown/ast.dart
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+// 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, String>{};
+
+  Element.empty(this.tag)
+    : children = null,
+      attributes = <String, String>{};
+
+  Element.tag(this.tag)
+    : children = [],
+      attributes = <String, String>{};
+
+  Element.text(this.tag, String text)
+    : children = [new Text(text)],
+      attributes = <String, 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);
+}