Be spec compliant. Fix issue 6870.

sdk/lib/_internal/compiler/implementation/world.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 dart2js;
 
 class World {
   final Compiler compiler;
   final Map<ClassElement, Set<ClassElement>> subtypes;
   final Map<ClassElement, Set<ClassElement>> typesImplementedBySubclasses;
@@ skipping 108 matching lines @@
   bool hasAnyUserDefinedGetter(Selector selector) {
     return userDefinedGetters.hasAnyElementMatchingSelector(selector);
   }
 
   bool hasAnyUserDefinedSetter(Selector selector) {
     return userDefinedSetters.hasAnyElementMatchingSelector(selector);
   }
 
   // Returns whether a subclass of [superclass] implements [type].
   bool hasAnySubclassThatImplements(ClassElement superclass, DartType type) {
-    Set<ClassElement> typesImplementedBySubclasses =
-          typesImplementedBySubclasses[superclass];
-    if (typesImplementedBySubclasses == null) return false;
-    return typesImplementedBySubclasses.contains(type.element);
+    Set<ClassElement> subclasses= typesImplementedBySubclasses[superclass];
+    if (subclasses == null) return false;
+    return subclasses.contains(type.element);
   }
 
   bool hasNoOverridingMember(Element element) {
     ClassElement cls = element.getEnclosingClass();
     Set<ClassElement> subclasses = compiler.world.subtypes[cls];
     // TODO(ngeoffray): Implement the full thing.
     return subclasses == null || subclasses.isEmpty;
   }
 
 
@@ skipping 78 matching lines @@
   final SourceString name;
 
   MemberSet(SourceString this.name) : elements = new Set<Element>();
 
   void add(Element element) {
     elements.add(element);
   }
 
   bool get isEmpty => elements.isEmpty;
 }