Use Entity instead of Element in ConstantValue

tests/compiler/dart2js/kernel/constructors_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.
 
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/elements/elements.dart';
 import 'package:test/test.dart';
 
 import 'helper.dart' show check;
 
@@ skipping 16 matching lines @@
   int x = 1;
 }
 
 main() {
   var a = new A();
   return a;
 }''';
     return check(code, lookup: defaultConstructorFor('A'));
   });
 
-test('redirecting constructor with field', () {
+  test('redirecting constructor with field', () {
     String code = '''
 class Foo {
   final int value;
   const Foo({int number: 0}) : this.poodle(number * 2);
   const Foo.poodle(this.value);
 }
 
 main() => new Foo(number: 3);
 ''';
     return check(code, lookup: defaultConstructorFor('Foo'));
   });
 }
 
 defaultConstructorFor(String className) => (Compiler compiler) {
       ClassElement clazz = compiler.mainApp.find(className);
       return clazz.lookupDefaultConstructor();
     };