Start moving the JavaScript backend related code into a separate library

lib/compiler/implementation/js_backend/js_backend.dart

Index: lib/compiler/implementation/js_backend/js_backend.dart
diff --git a/lib/compiler/implementation/dart_backend/dart_backend.dart b/lib/compiler/implementation/js_backend/js_backend.dart
similarity index 57%
copy from lib/compiler/implementation/dart_backend/dart_backend.dart
copy to lib/compiler/implementation/js_backend/js_backend.dart
index 9f787e975a312cf47dd6214b6c4f83033904297b..e495570015dee265ad3c487bd0164dc235cd86ce 100644
--- a/lib/compiler/implementation/dart_backend/dart_backend.dart
+++ b/lib/compiler/implementation/js_backend/js_backend.dart
@@ -2,15 +2,19 @@
 // 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.
 
-#library('dart_backend');
+#library('js_backend');
 
+#import('../../compiler.dart', prefix: 'api');
 #import('../elements/elements.dart');
 #import('../leg.dart');
+#import('../native_handler.dart', prefix: 'native');
+#import('../scanner/scannerlib.dart');
+#import('../source_file.dart');
+#import('../source_map_builder.dart');
+#import('../ssa/ssa.dart');
 #import('../tree/tree.dart');
 #import('../util/util.dart');
 
 #source('backend.dart');
 #source('emitter.dart');
-#source('renamer.dart');
-#source('placeholder.dart');
-#source('placeholder_collector.dart');
+#source('native_emitter.dart');