Encapsulate the index based maps for entities.

pkg/compiler/lib/src/kernel/elements.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.
-
-/// Entity interfaces for modeling elements derived from Kernel IR.
-
-import '../elements/entities.dart';
-
-abstract class IndexedLibrary implements LibraryEntity {
-  /// Library index used for fast lookup in [KernelToElementMapBase].
-  int get libraryIndex;
-}
-
-abstract class IndexedClass implements ClassEntity {
-  /// Class index used for fast lookup in [KernelToElementMapBase].
-  int get classIndex;
-}
-
-abstract class IndexedMember implements MemberEntity {
-  /// Member index used for fast lookup in [KernelToElementMapBase].
-  int get memberIndex;
-}
-
-abstract class IndexedFunction implements IndexedMember, FunctionEntity {}
-
-abstract class IndexedConstructor
-    implements IndexedFunction, ConstructorEntity {}
-
-abstract class IndexedField implements IndexedMember, FieldEntity {}
-
-abstract class IndexedTypeVariable implements TypeVariableEntity {
-  /// Type variable index used for fast lookup in [KernelToElementMapBase].
-  int get typeVariableIndex;
-}
-
-abstract class IndexedTypedef implements TypedefEntity {
-  /// Typedef index used for fast lookup in [KernelToElementMapBase].
-  int get typedefIndex;
-}