Split js_backend into multiple libraries

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

Index: pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
index 409c44dff81307d778ff3e46c1ccdc69fe75bd80..c8600e1206867d92763dee167bdc3399bf4c8bb0 100644
--- a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
@@ -2,7 +2,17 @@
 // 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 '../compiler.dart' show Compiler;
+import '../dart_types.dart';
+import '../elements/elements.dart';
+import '../js/js.dart' as jsAst;
+import '../js/js.dart' show js;
+import '../ssa/codegen.dart' show SsaCodeGenerator;
+import '../ssa/nodes.dart' show HTypeConversion, HInstruction;
+import '../universe/call_structure.dart' show CallStructure;
+import '../universe/use.dart'
+    show DynamicUse, StaticUse, StaticUseKind, TypeUse, TypeUseKind;
+import 'backend.dart';
 
 class CheckedModeHelper {
   final String name;