Fix receiver refining optimization in the presence of null: any Object method can be called on the …

sdk/lib/_internal/compiler/implementation/universe/function_set.dart

Index: sdk/lib/_internal/compiler/implementation/universe/function_set.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/universe/function_set.dart	(revision 26345)
+++ sdk/lib/_internal/compiler/implementation/universe/function_set.dart	(working copy)
@@ -220,6 +220,9 @@
               : [cls];
         })
         .map((cls) {
+          if (compiler.backend.isNullImplementation(cls)) {
+            return const TypeMask.empty();
+          }
           return compiler.world.hasSubclasses(cls)
               ? new TypeMask.nonNullSubclass(cls.rawType)
               : new TypeMask.nonNullExact(cls.rawType);