Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

 // Copyright (c) 2013, 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.
 
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../universe/use.dart' show StaticUse;
 import '../universe/world_impact.dart'
@@ skipping 36 matching lines @@
  *     const Component<FancyButton>('x-fancy-button').register();
  *
  * In these cases we conservatively generate all viable entries in the table.
  */
 class CustomElementsAnalysis {
   final JavaScriptBackend backend;
   final Compiler compiler;
   final CustomElementsAnalysisJoin resolutionJoin;
   final CustomElementsAnalysisJoin codegenJoin;
   bool fetchedTableAccessorMethod = false;
-  Element tableAccessorMethod;
+  MethodElement tableAccessorMethod;
 
   CustomElementsAnalysis(JavaScriptBackend backend)
       : this.backend = backend,
         this.compiler = backend.compiler,
         resolutionJoin = new CustomElementsAnalysisJoin(backend),
         codegenJoin = new CustomElementsAnalysisJoin(backend) {
     // TODO(sra): Remove this work-around.  We should mark allClassesSelected in
     // both joins only when we see a construct generating an unknown [Type] but
     // we can't currently recognize all cases.  In particular, the work-around
     // for the unimplemented `ClassMirror.reflectedType` is not recognizable.
@@ skipping 142 matching lines @@
           result.add(member);
         }
       }
     }
 
     classElement.forEachMember(selectGenerativeConstructors,
         includeBackendMembers: false, includeSuperAndInjectedMembers: false);
     return result;
   }
 }