Fix type tests for null.

sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart
index 9241edc59c2637291b36fad24519704bc176ebfd..accc5060590f63ad09de16b0825137671f88e02d 100644
--- a/sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/types/simple_types_inferrer.dart
@@ -2053,7 +2053,8 @@ class SimpleTypeInferrerVisitor extends ResolvedVisitor<TypeMask> {
       sideEffects.add(nativeBehavior.sideEffects);
       return inferrer.typeOfNativeBehavior(nativeBehavior);
     } else if (name == const SourceString('JS_OPERATOR_IS_PREFIX')
-               || name == const SourceString('JS_OPERATOR_AS_PREFIX')) {
+               || name == const SourceString('JS_OPERATOR_AS_PREFIX')
+               || name == const SourceString('JS_OBJECT_CLASS_NAME')) {
       return inferrer.stringType;
     } else {
       sideEffects.setAllSideEffects();