Change js_emitter parts into libraries

pkg/compiler/lib/src/js_emitter/native_generator.dart

 // Copyright (c) 2014, 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 dart2js.js_emitter;
+library dart2js.js_emitter.native_generator;
+
+import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
+
+import '../js/js.dart' as jsAst;
+import '../js/js.dart' show js;
+import '../js_backend/js_backend.dart'
+    show
+    JavaScriptBackend;
+
+import 'model.dart';
 
 class NativeGenerator {
   static bool needsIsolateAffinityTagInitialization(JavaScriptBackend backend) {
     return backend.needToInitializeIsolateAffinityTag;
   }
 
   /// Generates the code for isolate affinity tags.
   ///
   /// Independently Dart programs on the same page must not interfer and
   /// this code sets up the variables needed to guarantee that behavior.
@@ skipping 185 matching lines @@
           'info': infoAccess,
           'constructor': constructorAccess,
           'subclassRead': subclassRead,
           'interceptorsByTagAccess': interceptorsByTagAccess,
           'leafTagsAccess': leafTagsAccess,
           'nativeSuperclassTagName': embeddedNames.NATIVE_SUPERCLASS_TAG_NAME,
           'allowNativesSubclassing': true
         });
   }
 }