Change TypeMask.base to be a ClassElement.

tests/compiler/dart2js/simple_inferrer_closure_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 'compiler_helper.dart';
 import 'parser_helper.dart' show buildSourceString;
 
 const String TEST = """
@@ skipping 127 matching lines @@
     checkReturn('returnDyn3', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnNum1', compiler.typesTask.numType);
 
     checkReturnInClass(String className, String methodName, type) {
       var cls = findElement(compiler, className);
       var element = cls.lookupLocalMember(buildSourceString(methodName));
       Expect.equals(type,
           typesInferrer.getReturnTypeOfElement(element).simplify(compiler));
     }
     var cls = findElement(compiler, 'A');
-    checkReturnInClass('A', 'foo', new TypeMask.nonNullExact(cls.rawType));
+    checkReturnInClass('A', 'foo', new TypeMask.nonNullExact(cls));
   }));
 }