Use Entity instead of Element in ConstantValue

tests/compiler/dart2js/kernel/visitor_test.dart

 // Copyright (c) 2016, 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.
 
 /// Test that the dart2js copy of [KernelVisitor] generates the expected IR as
 /// defined by kernel spec-mode test files.
 
 import 'dart:io';
 import 'dart:async';
 import 'package:compiler/src/compiler.dart' show Compiler;
@@ skipping 10 matching lines @@
 import '../memory_compiler.dart';
 
 const String TESTCASE_DIR = 'pkg/kernel/testcases/';
 
 const List<String> SKIP_TESTS = const <String>[
   /// The test expects an unpatched api.
   'external',
 ];
 
 main(List<String> arguments) async {
-
   Directory directory = new Directory('${TESTCASE_DIR}/input');
   for (FileSystemEntity file in directory.listSync()) {
     if (file is File && file.path.endsWith('.dart')) {
       String name = pathlib.basenameWithoutExtension(file.path);
       bool selected = arguments.contains(name);
       if (!selected) {
         if (arguments.isNotEmpty) continue;
         if (SKIP_TESTS.contains(name)) continue;
       }
 
@@ skipping 36 matching lines @@
   await compiler.setupSdk();
 
   // The visitor no longer enqueues elements that are not reachable from the
   // program. The mixin-full resolution transform run by the test expects to
   // find dart.core::Iterator.
   var core = await compiler.libraryLoader.loadLibrary(Uri.parse('dart:core'));
   var cls = core.implementation.localLookup('Iterator');
   cls.ensureResolved(compiler.resolution);
   return compiler;
 }