Move Null class out of the VM isolate.

tests/lib/lib.status

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 [ $compiler == dart2js && ($runtime == d8 || $runtime == drt) ]
 convert/chunked_conversion_utf87_test: Pass, Fail # v8 bug: Issue 12293
 typed_data/byte_data_test: Pass, Fail # v8 bug: Issue 12293
 
 [ $compiler == dart2js ]
 math/*: Skip
@@ skipping 83 matching lines @@
 [ $runtime == vm ]
 async/timer_not_available_test: Fail, OK # only meant to test when there is no way to
                                          # 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.
 
 [ $runtime == vm || ($compiler == none && $runtime == drt) ]
 async/run_async3_test: Fail # _enqueueImmediate runs after Timer. http://dartbug.com/9001.
 mirrors/parameter_test: Fail # http://dartbug.com/11567
 mirrors/operator_test: Fail # http://dartbug.com/11944
-mirrors/null_test: Fail # Issue 12128
 
 [ $compiler == none && $runtime == drt ]
 async/timer_isolate_test: Skip # See Issue 4997
 async/timer_not_available_test: Skip # only meant to test when there is no way to
                                      # implement timer (currently only in d8)
 mirrors/library_uri_io_test: Skip # Not intended for drt as it uses dart:io.
 mirrors/local_isolate_test: Skip # http://dartbug.com/12188
 async/run_async6_test: Fail # Issue 10910
 
 [ $compiler == none && $runtime == drt && $system == windows ]
 async/multiple_timer_test: Fail, Pass # See Issue 10982
 async/timer_test: Fail, Pass # See Issue 10982
 
 [ $arch == mips ]
 *: Skip
 
 [ $arch == simmips ]
 convert/chunked_conversion_utf88_test: Pass, Slow # Issue 12025.
 convert/streamed_conversion_json_utf8_decode_test: Pass, Slow