Split js_backend into multiple libraries

pkg/compiler/lib/src/js_backend/type_variable_handler.dart

Index: pkg/compiler/lib/src/js_backend/type_variable_handler.dart
diff --git a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
index faf8539e04d80b5bb455dbf323291792540bb2c1..334a648a0fde6c2eab596a317c16f4f09affdbee 100644
--- a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
+++ b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
@@ -2,7 +2,23 @@
 // 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.
 
-part of js_backend;
+import '../common.dart';
+import '../common/registry.dart' show EagerRegistry, Registry;
+import '../compiler.dart' show Compiler;
+import '../constants/expressions.dart';
+import '../constants/values.dart';
+import '../dart_types.dart';
+import '../elements/elements.dart';
+import '../enqueue.dart' show Enqueuer, ResolutionEnqueuer;
+import '../js/js.dart' as jsAst;
+import '../js_emitter/js_emitter.dart'
+    show CodeEmitterTask, MetadataCollector, Placeholder;
+import '../universe/call_structure.dart' show CallStructure;
+import '../universe/use.dart'
+    show DynamicUse, StaticUse, StaticUseKind, TypeUse, TypeUseKind;
+import '../util/util.dart';
+
+import 'backend.dart';
 
 /**
  * Handles construction of TypeVariable constants needed at runtime.