Move remaining helpers to BackendHelpers

pkg/compiler/lib/src/native/native.dart

 // Copyright (c) 2012, 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.
 
 library native;
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
 import '../common/backend_api.dart' show
@@ skipping 12 matching lines @@
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer;
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
     BaseClassElementX,
     ElementX,
     FunctionElementX,
     LibraryElementX;
 import '../js/js.dart' as js;
+import '../js_backend/backend_helpers.dart' show
+    BackendHelpers;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show
     CodeEmitterTask,
     NativeEmitter;
 import '../parser/listener.dart' show
     Listener;
 import '../parser/element_listener.dart' show
     ElementListener;
 import '../ssa/ssa.dart';
 import '../tokens/token.dart' show
@@ skipping 26 matching lines @@
       || libraryName == 'dart:svg'
       || libraryName == 'dart:_native_typed_data'
       || libraryName == 'dart:web_audio'
       || libraryName == 'dart:web_gl'
       || libraryName == 'dart:web_sql'
       || compiler.allowNativeExtensions) {
     return true;
   }
   return false;
 }