Make RegExp's constructor non-const.

sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.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.
 
 part of markdown;
 
 String renderToHtml(List<Node> nodes) => new HtmlRenderer().render(nodes);
 
 /// Translates a parsed AST to HTML.
 class HtmlRenderer implements NodeVisitor {
-  static const _BLOCK_TAGS = const RegExp(
+  static const _BLOCK_TAGS = new RegExp(
       'blockquote|h1|h2|h3|h4|h5|h6|hr|p|pre');
 
   StringBuffer buffer;
 
   HtmlRenderer();
 
   String render(List<Node> nodes) {
     buffer = new StringBuffer();
 
     for (final node in nodes) node.accept(this);
@@ skipping 30 matching lines @@
     } else {
       buffer.add('>');
       return true;
     }
   }
 
   void visitElementAfter(Element element) {
     buffer.add('</${element.tag}>');
   }
 }