Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_backend/backend_helpers.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.
 
 library dart2js.js_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Uris;
 import '../compiler.dart' show Compiler;
 import '../core_types.dart' show CommonElements, ElementEnvironment;
@@ skipping 37 matching lines @@
 
   ClassEntity _findInterceptorsClass(String name) =>
       _findClass(interceptorsLibrary, name);
 
   FunctionEntity _findInterceptorsFunction(String name) =>
       _findLibraryMember(interceptorsLibrary, name);
 
   ClassEntity _findHelperClass(String name) =>
       _findClass(jsHelperLibrary, name);
 
+  // TODO(johnniwinther): Avoid the need for this (from [CheckedModeHelper]).
+  FunctionEntity findHelperFunction(String name) => _findHelperFunction(name);
+
   FunctionEntity _findHelperFunction(String name) =>
       _findLibraryMember(jsHelperLibrary, name);
 
   ClassEntity _findAsyncHelperClass(String name) =>
       _findClass(asyncLibrary, name);
 
   FunctionEntity _findAsyncHelperFunction(String name) =>
       _findLibraryMember(asyncLibrary, name);
 
   FunctionEntity _findMirrorsFunction(String name) {
@@ skipping 729 matching lines @@
   /// The class for patch annotations defined in dart:_js_helper.
   ClassEntity get patchAnnotationClass =>
       _patchAnnotationClass ??= _findHelperClass('_Patch');
 
   ClassEntity _nativeAnnotationClass;
 
   /// The class for native annotations defined in dart:_js_helper.
   ClassEntity get nativeAnnotationClass =>
       _nativeAnnotationClass ??= _findHelperClass('Native');
 }