Big merge from experimental to bleeding edge.

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 {
@@ skipping 21 matching lines @@
     if (!buffer.isEmpty &&
         _BLOCK_TAGS.firstMatch(element.tag) != null) {
       buffer.add('\n');
     }
 
     buffer.add('<${element.tag}');
 
     // Sort the keys so that we generate stable output.
     // TODO(rnystrom): This assumes keys returns a fresh mutable
     // collection.
-    final attributeNames = element.attributes.keys;
+    final attributeNames = element.attributes.keys.toList();
     attributeNames.sort((a, b) => a.compareTo(b));
     for (final name in attributeNames) {
       buffer.add(' $name="${element.attributes[name]}"');
     }
 
     if (element.isEmpty) {
       // Empty element like <hr/>.
       buffer.add(' />');
       return false;
     } else {
       buffer.add('>');
       return true;
     }
   }
 
   void visitElementAfter(Element element) {
     buffer.add('</${element.tag}>');
   }
 }