Start resolving classes and default constructors.

frog/leg/scanner/class_element_parser.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.
 
 class ClassElementParser extends PartialParser {
   ClassElementParser(Listener listener) : super(listener);
 
   Token parseClassBody(Token token) => fullParseClassBody(token);
 }
 
 class PartialClassElement extends ClassElement {
   final Token beginToken;
   final Token endToken;
-  ClassNode node;
   Link<Element> members;
 
   PartialClassElement(SourceString name,
                       Token this.beginToken, Token this.endToken)
     : super(name);
 
   ClassNode parseNode(Canceler canceler, Logger logger) {
+    if (node != null) return node;
     NodeListener listener = new NodeListener(canceler, logger);
     Parser parser = new ClassElementParser(listener);
     Token token = parser.parseClass(beginToken);
     assert(token === endToken.next);
-    ClassNode node = listener.popNode();
+    node = listener.popNode();
     assert(listener.nodes.isEmpty());
     assert(listener.topLevelElements.isEmpty());
     return node;
   }
 }