Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index a1761209eae85b8c523008cb814b5b34e3f58e84..e6e5a5d4faf752bc5c0ffdd7f04806f3e228feef 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -29,7 +29,7 @@ abstract class RuntimeTypes {
   Iterable<ClassElement> get classesUsingTypeVariableExpression;
 
   void registerClassUsingTypeVariableExpression(ClassElement cls);
-  void registerRtiDependency(Element element, Element dependency);
+  void registerRtiDependency(ClassElement element, ClassElement dependency);
   void registerTypeVariableBoundsSubtypeCheck(
       ResolutionDartType typeArgument, ResolutionDartType bound);
 
@@ -142,9 +142,9 @@ class _RuntimeTypes implements RuntimeTypes {
   }
 
   @override
-  void registerRtiDependency(Element element, Element dependency) {
+  void registerRtiDependency(ClassElement element, ClassElement dependency) {
     // We're not dealing with typedef for now.
-    if (element == null || !element.isClass || !dependency.isClass) return;
+    assert(element != null);
     Set<ClassElement> classes =
         rtiDependencies.putIfAbsent(element, () => new Set<ClassElement>());
     classes.add(dependency);