Change js_emitter parts into libraries

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

Index: pkg/compiler/lib/src/js_emitter/native_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/native_generator.dart b/pkg/compiler/lib/src/js_emitter/native_generator.dart
index 4dfa36c540edaffd0befab509ad9046199160fe9..83b28c67816ac8abdc47c1e74bb768adb69b6a2c 100644
--- a/pkg/compiler/lib/src/js_emitter/native_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/native_generator.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 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) {