Remove BackendClasses and JavaScriptBackendClasses.

pkg/compiler/lib/src/universe/function_set.dart

Index: pkg/compiler/lib/src/universe/function_set.dart
diff --git a/pkg/compiler/lib/src/universe/function_set.dart b/pkg/compiler/lib/src/universe/function_set.dart
index fa136fbe5ec4cfcf4bd4393761ed2b216b9d6192..bb7dd4e0260565e55930aae3ff414025187345eb 100644
--- a/pkg/compiler/lib/src/universe/function_set.dart
+++ b/pkg/compiler/lib/src/universe/function_set.dart
@@ -297,7 +297,7 @@ class FullFunctionSetQuery implements FunctionSetQuery {
           ClassEntity cls = element.enclosingClass;
           return [cls]..addAll(closedWorld.mixinUsesOf(cls));
         }).map((cls) {
-          if (closedWorld.backendClasses.nullClass == cls) {
+          if (closedWorld.commonElements.jsNullClass == cls) {
             return const TypeMask.empty();
           } else if (closedWorld.isInstantiated(cls)) {
             return new TypeMask.nonNullSubclass(cls, closedWorld);