Reorganize dartdoc to new package layout.

pkg/dartdoc/lib/markdown.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.
 
 /// Parses text in a markdown-like format and renders to HTML.
 #library('markdown');
 
-#source('ast.dart');
-#source('block_parser.dart');
-#source('html_renderer.dart');
-#source('inline_parser.dart');
+// TODO(rnystrom): Use "package:" URL (#4968).
+#source('src/markdown/ast.dart');
+#source('src/markdown/block_parser.dart');
+#source('src/markdown/html_renderer.dart');
+#source('src/markdown/inline_parser.dart');
 
 /// Converts the given string of markdown to HTML.
 String markdownToHtml(String markdown) {
   final document = new Document();
 
   final lines = markdown.split('\n');
   document.parseRefLinks(lines);
   final blocks = document.parseLines(lines);
   return renderToHtml(blocks);
 }
@@ skipping 83 matching lines @@
   /// `<em>this <strong>is</strong> a</em> <code>markdown</code>`.
   List<Node> parseInline(String text) => new InlineParser(text, this).parse();
 }
 
 class Link {
   final String id;
   final String url;
   final String title;
   Link(this.id, this.url, this.title);
 }