Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

Index: pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
index 613abf9fb86d7f097dcbeb9d3e85c5fcb7f8fb8c..044cdd5b5ba32691266879e3d6888886f7426b95 100644
--- a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
@@ -22,7 +22,8 @@ class CheckedModeHelper {
     JavaScriptBackend backend = compiler.backend;
     // TODO(johnniwinther): Refactor this to avoid looking up directly in the
     // js helper library but instead access helpers directly on backend helpers.
-    MethodElement method = backend.helpers.jsHelperLibrary.find(name);
+    LibraryElement jsHelperLibrary = backend.helpers.jsHelperLibrary;
+    MethodElement method = jsHelperLibrary.find(name);
     return new StaticUse.staticInvoke(method, callStructure);
   }