Make elements/names.dart its own library

pkg/compiler/lib/src/kernel/element_map.dart

 // Copyright (c) 2017, 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.kernel.element_map;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart';
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/constructors.dart';
 import '../constants/evaluation.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../common_elements.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
+import '../elements/names.dart';
 import '../elements/types.dart';
 import '../environment.dart';
 import '../frontend_strategy.dart';
 import '../js_backend/constant_system_javascript.dart';
 import '../js_backend/native_data.dart';
 import '../js_backend/no_such_method_registry.dart';
 import '../native/native.dart' as native;
 import '../native/resolver.dart';
 import '../ordered_typeset.dart';
 import '../ssa/kernel_impact.dart';
@@ skipping 1271 matching lines @@
   }
 
   InterfaceType getMixinTypeForClass(KClass cls) {
     _KClassEnv env = elementMap._classEnvs[cls.classIndex];
     ir.Supertype mixedInType = env.cls.mixedInType;
     if (mixedInType == null) return null;
     return elementMap.createInterfaceType(
         mixedInType.classNode, mixedInType.typeArguments);
   }
 }