Merge CoreTypes and CoreClasses into CommonElements.

tests/compiler/dart2js/expect_annotations_test.dart

 // Copyright (c) 2015, 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/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/types/types.dart';
@@ skipping 88 matching lines @@
         testTypeMatch(
             method, expectedParameterType, expectedReturnType, inferrer);
       } else if (expectAssumeDynamic) {
         testTypeMatch(
             method, closedWorld.commonMasks.dynamicType, null, inferrer);
       }
     }
 
     TypeMask jsStringType = closedWorld.commonMasks.stringType;
     TypeMask jsIntType = closedWorld.commonMasks.intType;
-    TypeMask coreStringType =
-        new TypeMask.subtype(compiler.coreClasses.stringClass, closedWorld);
+    TypeMask coreStringType = new TypeMask.subtype(
+        closedWorld.commonElements.stringClass, closedWorld);
 
     test('method');
     test('methodAssumeDynamic', expectAssumeDynamic: true);
     test('methodTrustTypeAnnotations',
         expectTrustTypeAnnotations: true, expectedParameterType: jsStringType);
     test('methodNoInline', expectNoInline: true);
     test('methodNoInlineTrustTypeAnnotations',
         expectNoInline: true,
         expectTrustTypeAnnotations: true,
         expectedParameterType: jsStringType,
         expectedReturnType: jsIntType);
     test('methodAssumeDynamicTrustTypeAnnotations',
         expectAssumeDynamic: true,
         expectTrustTypeAnnotations: true,
         expectedParameterType: coreStringType);
   });
 }