Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/js_backend/namer.dart

 // Copyright (c) 2011, 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 js_backend.namer;
 
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' show JsGetName;
 
@@ skipping 1355 matching lines @@
       names.add("x");
     }
     // Sort the names of the classes after abbreviating them to ensure
     // the suffix is stable and predictable for the suggested names.
     names.sort();
     return names.join();
   }
 
   /// Property name used for `getInterceptor` or one of its specializations.
   jsAst.Name nameForGetInterceptor(Iterable<ClassEntity> classes) {
-    FunctionElement getInterceptor = helpers.getInterceptorMethod;
+    MethodElement getInterceptor = helpers.getInterceptorMethod;
     if (classes.contains(helpers.jsInterceptorClass)) {
       // If the base Interceptor class is in the set of intercepted classes, we
       // need to go through the generic getInterceptorMethod, since any subclass
       // of the base Interceptor could match.
       // The unspecialized getInterceptor method can also be accessed through
       // its element, so we treat this as a user-space global instead of an
       // internal global.
       return _disambiguateGlobal(getInterceptor);
     }
     String suffix = suffixForGetInterceptor(classes);
@@ skipping 805 matching lines @@
   void addSuggestion(String original, String suggestion) {
     assert(!_suggestedNames.containsKey(original));
     _suggestedNames[original] = suggestion;
   }
 
   bool hasSuggestion(String original) => _suggestedNames.containsKey(original);
   bool isSuggestion(String candidate) {
     return _suggestedNames.containsValue(candidate);
   }
 }