Move markdown library.

samples/markdown/html_renderer.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.
 
 String renderToHtml(List<Node> nodes) => new HtmlRenderer().render(nodes);
 
 /// Translates a parsed AST to HTML.
 class HtmlRenderer implements NodeVisitor {
   static final _BLOCK_TAGS = const RegExp(
       'blockquote|h1|h2|h3|h4|h5|h6|hr|p|pre');
@@ skipping 34 matching lines @@
     } else {
       buffer.add('>');
       return true;
     }
   }
 
   void visitElementAfter(Element element) {
     buffer.add('</${element.tag}>');
   }
 }