Create type masks lazily.

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
     '../../../sdk/lib/_internal/compiler/implementation/types/types.dart'
     show TypeMask;
 
@@ skipping 712 matching lines @@
     checkFactoryConstructor(String className, String factoryName) {
       var cls = findElement(compiler, className);
       var element = cls.localLookup(buildSourceString(factoryName));
       Expect.equals(new TypeMask.nonNullExact(cls.rawType),
                     typesInferrer.getReturnTypeOfElement(element));
     }
     checkFactoryConstructor('A', '');
 
     checkReturn('testCascade1', typesTask.growableListType);
     checkReturn('testCascade2', new TypeMask.nonNullExact(
-        typesTask.rawTypeOf(findElement(compiler, 'CascadeHelper'))));
+        findElement(compiler, 'CascadeHelper').rawType));
     checkReturn('testSpecialization1', typesTask.numType);
     checkReturn('testSpecialization2', typesTask.dynamicType);
     checkReturn('testSpecialization3', typesTask.intType.nullable());
   }));
 }