Make elements/names.dart its own library

tests/compiler/dart2js/gvn_dynamic_field_get_test.dart

 // Copyright (c) 2012, 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 dart2js gvns dynamic getters that don't have side
 // effects.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
+import 'package:compiler/src/elements/names.dart';
 import 'package:compiler/src/universe/selector.dart' show Selector;
 
 const String TEST = r"""
 class A {
   var foo;
   bar(a) {
     return a.foo + a.foo;
   }
 }
 
@@ skipping 13 matching lines @@
         var cls = findElement(compiler, 'A');
         Expect.isNotNull(cls);
         String name = 'foo';
         var element = cls.lookupLocalMember(name);
         Expect.isNotNull(element);
         Selector selector = new Selector.getter(new PublicName(name));
         Expect.isFalse(compiler.resolutionWorldBuilder.closedWorldForTesting
             .hasAnyUserDefinedGetter(selector, null));
       }));
 }