Fix type tests for null.

sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart
index fb06bf3c958c9567d748eb425c4e5b66b4feb1f6..19148962d9e800b742a6f2252da3ce1db044aaf4 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/foreign_helper.dart
@@ -158,6 +158,9 @@ String JS_OPERATOR_IS_PREFIX() {}
  */
 String JS_OPERATOR_AS_PREFIX() {}
 
+/// Returns the name of the class `Object` in the generated code.
+String JS_OBJECT_CLASS_NAME() {}
+
 /**
  * Returns the field name used for determining if an object or its
  * interceptor has JavaScript indexing behavior.