Merge request - fixes dart2js async problems.

pkg/compiler/lib/src/dart_types.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.
 
 library dart_types;
 
 import 'dart:math' show min;
 
 import 'core_types.dart';
 import 'dart2jslib.dart' show Compiler, invariant, Script, Message;
@@ skipping 457 matching lines @@
   bool get isEnumType => element.isEnumClass;
 
   InterfaceType createInstantiation(List<DartType> newTypeArguments) {
     return new InterfaceType(element, newTypeArguments);
   }
 
   /**
    * Returns the type as an instance of class [other], if possible, null
    * otherwise.
    */
-  DartType asInstanceOf(ClassElement other) {
+  InterfaceType asInstanceOf(ClassElement other) {
     other = other.declaration;
     if (element == other) return this;
     InterfaceType supertype = element.asInstanceOf(other);
     if (supertype != null) {
       List<DartType> arguments = Types.substTypes(supertype.typeArguments,
                                                   typeArguments,
                                                   element.typeVariables);
       return new InterfaceType(supertype.element, arguments);
     }
     return null;
@@ skipping 1327 matching lines @@
         }
         namedParameterTypes[index].accept(this, namedParameters[index]);
         needsComma = true;
       }
       sb.write('}');
     }
     sb.write(')');
   }
 }