Mark null_test as failing on Dartium as well.

tests/lib/lib.status

Index: tests/lib/lib.status
===================================================================
--- tests/lib/lib.status	(revision 25701)
+++ tests/lib/lib.status	(working copy)
@@ -87,7 +87,6 @@
                                          # implement timer (currently only in d8)
 async/timer_isolate_test: Skip # See Issue 4997
 mirrors/native_class_test: Fail, OK # This test is meant to run in a browser.
-mirrors/null_test: Fail # Issue 12128
 
 [ $runtime == vm || ($compiler == none && $runtime == drt) ]
 async/run_async3_test: Fail # _enqueueImmediate runs after Timer. http://dartbug.com/9001.
@@ -95,6 +94,7 @@
 mirrors/parameter_test: Fail # http://dartbug.com/11567
 mirrors/operator_test: Fail # http://dartbug.com/11944
 mirrors/local_isolate_test: Fail # http://dartbug.com/12026
+mirrors/null_test: Fail # Issue 12128
 
 [ $compiler == none && $runtime == drt ]
 async/timer_isolate_test: Skip # See Issue 4997