Use entities in BackendClasses

pkg/compiler/lib/src/universe/function_set.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 universe.function_set;
 
 import '../common/names.dart' show Identifiers, Selectors;
 import '../elements/elements.dart' show MemberElement;
 import '../elements/entities.dart';
 import '../types/types.dart';
@@ skipping 280 matching lines @@
   @override
   TypeMask computeMask(ClosedWorld closedWorld) {
     assert(closedWorld
         .hasAnyStrictSubclass(closedWorld.commonElements.objectClass));
     if (_mask != null) return _mask;
     return _mask = new TypeMask.unionOf(
         functions.expand((MemberElement element) {
           ClassEntity cls = element.enclosingClass;
           return [cls]..addAll(closedWorld.mixinUsesOf(cls));
         }).map((cls) {
-          if (closedWorld.backendClasses.nullImplementation == cls) {
+          if (closedWorld.backendClasses.nullClass == cls) {
             return const TypeMask.empty();
           } else if (closedWorld.isInstantiated(cls)) {
             return new TypeMask.nonNullSubclass(cls, closedWorld);
           } else {
             // TODO(johnniwinther): Avoid the need for this case.
             return const TypeMask.empty();
           }
         }),
         closedWorld);
   }
 }