Use index in J/KTypeVariable and fix remaining inequivalence

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].
@@ skipping 10 matching lines @@
   int get memberIndex;
 }
 
 abstract class IndexedFunction implements IndexedMember, FunctionEntity {}
 
 abstract class IndexedConstructor
     implements IndexedFunction, ConstructorEntity {}
 
 abstract class IndexedField implements IndexedMember, FieldEntity {}
 
-// TODO(johnniwinther): Add and use [typeVariableIndex].
-abstract class IndexedTypeVariable implements TypeVariableEntity {}
+abstract class IndexedTypeVariable implements TypeVariableEntity {
+  /// Type variable index used for fast lookup in [KernelToElementMapBase].
+  int get typeVariableIndex;
+}