Use entities in the TypeMask interface.

tests/compiler/dart2js/simple_inferrer_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/types/types.dart' show TypeMask;
 import 'type_mask_test_helper.dart';
 
 import 'compiler_helper.dart';
@@ skipping 854 matching lines @@
         checkFactoryConstructor(String className, String factoryName) {
           var cls = findElement(compiler, className);
           var element = cls.localLookup(factoryName);
           Expect.equals(new TypeMask.nonNullExact(cls, closedWorld),
               typesInferrer.getReturnTypeOfElement(element));
         }
 
         checkFactoryConstructor('A', '');
 
         checkReturn('testCascade1', commonMasks.growableListType);
-        checkReturn(
-            'testCascade2',
-            new TypeMask.nonNullExact(
-                findElement(compiler, 'CascadeHelper'), closedWorld));
+        ClassElement clsCascadeHelper = findElement(compiler, 'CascadeHelper');
+        checkReturn('testCascade2',
+            new TypeMask.nonNullExact(clsCascadeHelper, closedWorld));
         checkReturn('testSpecialization1', commonMasks.numType);
         checkReturn('testSpecialization2', commonMasks.dynamicType);
         checkReturn('testSpecialization3', commonMasks.uint31Type.nullable());
         checkReturn('testReturnNull1', commonMasks.nullType);
         checkReturn('testReturnNull2', commonMasks.nullType);
         checkReturn('testReturnNull3', commonMasks.dynamicType);
         checkReturn('testReturnNull4', commonMasks.nullType);
         checkReturn('testReturnNull5', commonMasks.nullType);
         checkReturn('testReturnNull6', commonMasks.dynamicType);
         checkReturn('testReturnNotEquals', commonMasks.boolType);
       }));
 }