Reorganize dartdoc to new package layout.

pkg/dartdoc/lib/src/markdown/block_parser.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.
 
 /// The line contains only whitespace or is empty.
 const _RE_EMPTY = const RegExp(@'^([ \t]*)$');
 
 /// A series of `=` or `-` (on the next line) define setext-style headers.
 const _RE_SETEXT = const RegExp(@'^((=+)|(-+))$');
 
@@ skipping 416 matching lines @@
     while (!BlockSyntax.isAtBlockEnd(parser)) {
       childLines.add(parser.current);
       parser.advance();
     }
 
     final contents = parser.document.parseInline(
         Strings.join(childLines, '\n'));
     return new Element('p', contents);
   }
 }