Use index in J/KTypeVariable and fix remaining inequivalence

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

Index: pkg/compiler/lib/src/kernel/elements.dart
diff --git a/pkg/compiler/lib/src/kernel/elements.dart b/pkg/compiler/lib/src/kernel/elements.dart
index c8f9a35559598531b27fdfa0685396f340e886c4..241448f65cdd13d1ec32d4f55c2cd6e8fb440b82 100644
--- a/pkg/compiler/lib/src/kernel/elements.dart
+++ b/pkg/compiler/lib/src/kernel/elements.dart
@@ -28,5 +28,7 @@ abstract class IndexedConstructor
 
 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;
+}