Move remaining helpers to BackendHelpers

tests/compiler/dart2js/minimal_resolution_test.dart

 // Copyright (c) 2015, 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.
 
 // Test that elements are not needlessly required by dart2js.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/enqueue.dart';
-import 'package:compiler/src/js_backend/js_backend.dart';
+import 'package:compiler/src/js_backend/backend_helpers.dart';
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 
 main() {
   asyncTest(() async {
     await analyze('main() {}');
     await analyze('main() => proxy;', proxyConstant: true);
     await analyze('@deprecated main() {}');
     await analyze('@deprecated main() => deprecated;', deprecatedClass: true);
     await analyze('main() => deprecated;', deprecatedClass: true);
@@ skipping 21 matching lines @@
   Compiler compiler = result.compiler;
   Expect.equals(proxyConstant, compiler.proxyConstant != null,
       "Unexpected computation of proxy constant.");
 
   checkInstantiated(
       compiler, compiler.coreLibrary.find('_Proxy'), proxyConstant);
   checkInstantiated(
       compiler, compiler.coreLibrary.find('Deprecated'), deprecatedClass);
 
   LibraryElement jsHelperLibrary =
-      compiler.libraryLoader.lookupLibrary(JavaScriptBackend.DART_JS_HELPER);
+      compiler.libraryLoader.lookupLibrary(BackendHelpers.DART_JS_HELPER);
   jsHelperLibrary.forEachLocalMember((Element element) {
     Uri uri = element.compilationUnit.script.resourceUri;
     if (element.isClass && uri.path.endsWith('annotations.dart')) {
       checkInstantiated(compiler, element, false);
     }
   });
 }