[VM] Use ^= (which allows "null") instead of RawCast
[VM] Use ^= (which allows "null") instead of RawCast

Change-Id: Ib22e6c38bb50128c6af83e044b42658aedbbe87d
Reviewed-on: https://dart-review.googlesource.com/31985
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Commit-Queue: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk