Change TypeMask.base to be a ClassElement.

tests/compiler/dart2js/simple_inferrer_try_catch_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;
 
 import 'compiler_helper.dart';
@@ skipping 165 matching lines @@
       Expect.equals(type,
           typesInferrer.getReturnTypeOfElement(element).simplify(compiler));
     }
 
     checkReturn('returnInt1', typesTask.intType);
     checkReturn('returnInt2', typesTask.intType);
     checkReturn('returnInt3', typesTask.intType);
     checkReturn('returnInt4', typesTask.intType);
     checkReturn('returnInt5', typesTask.intType);
     checkReturn('returnInt6',
-        new TypeMask.nonNullSubtype(compiler.intClass.rawType));
+        new TypeMask.nonNullSubtype(compiler.intClass));
 
     var subclassOfInterceptor =
         findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
     checkReturn('returnDyn1', subclassOfInterceptor);
     checkReturn('returnDyn2', subclassOfInterceptor);
     checkReturn('returnDyn3', subclassOfInterceptor);
     checkReturn('returnDyn4', subclassOfInterceptor);
     checkReturn('returnDyn5', subclassOfInterceptor);
     checkReturn('returnDyn6', typesTask.dynamicType);
   }));
 }