Make elements/names.dart its own library

pkg/compiler/lib/src/elements/common.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.
 
 /// Mixins that implement convenience methods on [Element] subclasses.
 
 library elements.common;
 
 import '../common/names.dart' show Identifiers, Names, Uris;
 import '../common_elements.dart' show CommonElements;
 import '../util/util.dart' show Link;
 import 'entities.dart';
 import 'elements.dart';
+import 'names.dart';
 import 'types.dart';
 import 'resolution_types.dart'
     show ResolutionDartType, ResolutionInterfaceType, ResolutionFunctionType;
 
 abstract class ElementCommon implements Element {
   @override
   bool get isLibrary => kind == ElementKind.LIBRARY;
 
   @override
   bool get isCompilationUnit => kind == ElementKind.COMPILATION_UNIT;
@@ skipping 648 matching lines @@
   @override
   bool get isBoolFromEnvironmentConstructor {
     return fromEnvironmentState == _FromEnvironmentState.BOOL;
   }
 
   @override
   bool get isStringFromEnvironmentConstructor {
     return fromEnvironmentState == _FromEnvironmentState.STRING;
   }
 }