Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

Index: pkg/compiler/lib/src/js_backend/patch_resolver.dart
diff --git a/pkg/compiler/lib/src/js_backend/patch_resolver.dart b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
index f7477fef57d8d3de9431fc3548018fd569fbab19..29e16d934c021e0772bb50d92e74eaad71234652 100644
--- a/pkg/compiler/lib/src/js_backend/patch_resolver.dart
+++ b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
@@ -42,6 +42,9 @@ class PatchResolverTask extends CompilerTask {
 
   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];
@@ -86,7 +89,7 @@ class PatchResolverTask extends CompilerTask {
             // 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, {