Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

 // Copyright (c) 2014, 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.patch_resolver;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
@@ skipping 24 matching lines @@
       element = patch;
     } else if (!compiler.backend.isJsInterop(element)) {
       reporter.reportErrorMessage(
           element, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
     }
     return element;
   }
 
   void checkMatchingPatchParameters(FunctionElement origin,
       List<Element> originParameters, List<Element> patchParameters) {
+    bool isUnnamedListConstructor = origin is ConstructorElement &&
+        compiler.commonElements.isUnnamedListConstructor(origin);
+
     assert(originParameters.length == patchParameters.length);
     for (int index = 0; index < originParameters.length; index++) {
       ParameterElementX originParameter = originParameters[index];
       ParameterElementX patchParameter = patchParameters[index];
       // TODO(johnniwinther): Remove the conditional patching when we never
       // resolve the same method twice.
       if (!originParameter.isPatched) {
         originParameter.applyPatch(patchParameter);
       } else {
         assert(invariant(origin, originParameter.patch == patchParameter,
@@ skipping 24 matching lines @@
         // elsewhere.
 
         // The node contains the type, so there is a potential overlap.
         // Therefore we only check the text if the types are identical.
         String originParameterText = originParameter.node.toString();
         String patchParameterText = patchParameter.node.toString();
         if (originParameterText != patchParameterText
             // We special case the list constructor because of the
             // optional parameter.
             &&
-            origin != compiler.commonElements.unnamedListConstructor) {
+            !isUnnamedListConstructor) {
           reporter.reportError(
               reporter.createMessage(
                   originParameter, MessageKind.PATCH_PARAMETER_MISMATCH, {
                 'methodName': origin.name,
                 'originParameter': originParameterText,
                 'patchParameter': patchParameterText
               }),
               <DiagnosticMessage>[
                 reporter.createMessage(
                     patchParameter,
@@ skipping 73 matching lines @@
           'originParameterCount': originSignature.optionalParameterCount,
           'patchParameterCount': patchSignature.optionalParameterCount
         });
       });
     } else {
       checkMatchingPatchParameters(origin, originSignature.optionalParameters,
           patchSignature.optionalParameters);
     }
   }
 }