Reorganize dartdoc to new package layout.

pkg/dartdoc/lib/src/markdown/ast.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.
 
 /// Base class for any AST item. Roughly corresponds to Node in the DOM. Will
 /// be either an Element or Text.
 abstract class Node {
   void accept(NodeVisitor visitor);
 }
 
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   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);
 }