Start allowing mixin application extensions.

sdk/lib/_internal/compiler/implementation/tree/nodes.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 tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
@@ skipping 183 matching lines @@
   VariableDefinitions asVariableDefinitions() => null;
   While asWhile() => null;
 
   bool isValidBreakTarget() => false;
   bool isValidContinueTarget() => false;
 }
 
 class ClassNode extends Node {
   final Modifiers modifiers;
   final Identifier name;
-  final TypeAnnotation superclass;
+  final Node superclass;
   final NodeList interfaces;
   final NodeList typeParameters;
   final NodeList body;
 
   // TODO(ahe, karlklose): the default keyword is not recorded.
   final TypeAnnotation defaultClause;
 
   final Token beginToken;
   final Token extendsKeyword;
   final Token endToken;
@@ skipping 1836 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send != null) return f(send.arguments.head);
 }