Encapsulate the index based maps for entities.

tests/compiler/dart2js/kernel/test_helpers.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.
 
 library dart2js.kernel.equivalence;
 
 import 'package:compiler/src/common/backend_api.dart';
 import 'package:compiler/src/common/resolution.dart';
 import 'package:compiler/src/common/work.dart';
 import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/constants/values.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/entities.dart';
 import 'package:compiler/src/elements/resolution_types.dart';
 import 'package:compiler/src/elements/types.dart';
 import 'package:compiler/src/enqueue.dart';
-import 'package:compiler/src/kernel/elements.dart';
-import 'package:compiler/src/kernel/kelements.dart' show KLocalFunction;
 import 'package:compiler/src/kernel/element_map.dart';
 import 'package:compiler/src/kernel/element_map_impl.dart';
+import 'package:compiler/src/kernel/indexed.dart';
+import 'package:compiler/src/kernel/kelements.dart' show KLocalFunction;
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/ssa/kernel_impact.dart';
 import 'package:compiler/src/universe/world_impact.dart';
 import 'package:compiler/src/util/util.dart';
 
 class KernelEquivalence {
   final WorldDeconstructionForTesting testing;
 
   /// Set of mixin applications assumed to be equivalent.
   ///
@@ skipping 275 matching lines @@
     for (ConstructorElement constructor in element.constructors) {
       if (!constructor.isRedirectingFactory) {
         return true;
       }
     }
     // The class cannot itself be instantiated.
     return false;
   }
   return true;
 }