[vm] Speculative fix for race in idle notifier shutdown (monitor use-after-free).
[vm] Speculative fix for race in idle notifier shutdown (monitor use-after-free).

Don't restart the idle notifier when removing a message handler.

Bug: https://github.com/dart-lang/sdk/issues/31979
Change-Id: I7e899807e3d71fa1e70be2921c8348372ed3b54d
Reviewed-on: https://dart-review.googlesource.com/39849
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

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