Make .enclosingClass always return the declaration

pkg/compiler/lib/src/resolution/constructors.dart

 // Copyright (c) 2015, 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 dart2js.resolution.constructors;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../constants/constructors.dart'
     show
@@ skipping 80 matching lines @@
   }
 
   void resolveFieldInitializer(SendSet init) {
     // init is of the form [this.]field = value.
     final Node selector = init.selector;
     final String name = selector.asIdentifier().source;
     // Lookup target field.
     Element target;
     FieldElement field;
     if (isFieldInitializer(init)) {
-      target = constructor.enclosingClass.lookupLocalMember(name);
+      // Use [enclosingElement] instead of [enclosingClass] to ensure lookup in
+      // patch class when necessary.
+      ClassElement cls = constructor.enclosingElement;
+      target = cls.lookupLocalMember(name);
       if (target == null) {
         reporter.reportErrorMessage(
             selector, MessageKind.CANNOT_RESOLVE, {'name': name});
         target = new ErroneousFieldElementX(
             selector.asIdentifier(), constructor.enclosingClass);
       } else if (target.kind != ElementKind.FIELD) {
         reporter.reportErrorMessage(
             selector, MessageKind.NOT_A_FIELD, {'fieldName': name});
         target = new ErroneousFieldElementX(
             selector.asIdentifier(), constructor.enclosingClass);
@@ skipping 770 matching lines @@
     // constructors.
     return null;
   }
   // TODO(johnniwinther): Use [Name] for lookup.
   ConstructorElement constructor = cls.lookupConstructor(constructorName);
   if (constructor != null) {
     constructor = constructor.declaration;
   }
   return constructor;
 }