Add tests for reflecting on null and Null.

tests/lib/lib.status

Index: tests/lib/lib.status
===================================================================
--- tests/lib/lib.status	(revision 25620)
+++ tests/lib/lib.status	(working copy)
@@ -8,6 +8,7 @@
 mirrors/method_mirror_properties_test: Fail # Issue 11861
 mirrors/method_mirror_returntype_test : Fail # Issue 11928
 mirrors/mirrors_test: Fail # TODO(ahe): I'm working on fixing this.
+mirrors/null_test : Fail # Issue 12129
 mirrors/library_uri_io_test: Skip # Not intended for dart2js as it uses dart:io.
 async/run_async3_test: Fail # _enqueueImmediate runs after Timer. http://dartbug.com/9002
 async/run_async4_test: Pass, Fail # no global exception handler in isolates. http://dartbug.com/9012
@@ -83,6 +84,7 @@
                                          # 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.