Split js_backend into multiple libraries

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

 // Copyright (c) 2013, 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.
 
-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.
  *
  * The support for custom elements the compiler builds a table that maps the
  * custom element class's [Type] to the interceptor for the class and the
  * constructor(s) for the class.
  *
  * We want the table to contain only the custom element classes used, and we
  * want to avoid resolving and compiling constructors that are not used since
@@ skipping 180 matching lines @@
         if (parameters.requiredParameterCount == 0) {
           result.add(member);
         }
       }
     }
     classElement.forEachMember(selectGenerativeConstructors,
         includeBackendMembers: false, includeSuperAndInjectedMembers: false);
     return result;
   }
 }