Split js_backend into multiple libraries

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

Index: pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
index 450b826c1f7733a5d13fc1cf2dde26b458ec9df9..17219457de77445fa3fc9768f902e4161c77b945 100644
--- a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
@@ -2,7 +2,15 @@
 // 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 '../constants/values.dart';
+import '../dart_types.dart';
+import '../elements/elements.dart';
+import '../enqueue.dart' show Enqueuer, ResolutionEnqueuer;
+import '../universe/use.dart'
+    show DynamicUse, StaticUse, StaticUseKind, TypeUse, TypeUseKind;
+
+import 'backend.dart';
 
 /**
  * Support for Custom Elements.