Refactor ConstantExpression/ConstantConstructor to use entities.

tests/compiler/dart2js/analyze_dart2js_helpers_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.
 
 library dart2js.analyze_helpers.test;
 
 import 'dart:io';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/compiler_new.dart' show Diagnostic;
@@ skipping 193 matching lines @@
       NodeList arguments,
       CallStructure callStructure,
       _) {
     checkAccess(node, constructor);
     apply(arguments);
   }
 
   @override
   void visitConstConstructorInvoke(
       NewExpression node, ConstructedConstantExpression constant, _) {
-    checkAccess(node, constant.target);
+    ConstructorElement constructor = constant.target;
+    checkAccess(node, constructor);
   }
 }