Do not handle Dart isolate messages if the isolate is being shut down

[jason-simmons]

This fixes a race that can happen if Dart invokes an isolate's message handling callback during isolate shutdown. The callback installed by Tonic will invoke a dispatcher set by the engine's DartIsolate::SetMessageHandlingTaskRunner, which will queue a message handling task to the designated task runner.

If the queued task runs after DartIsolate::Shutdown has been called, then Tonic's DartMessageHandler::OnHandleMessage will fail when it tries to enter the isolate.

DartIsolate will set a shutdown flag on the DartState when Dart invokes the isolate's shutdown callback. Tonic can avoid this race by checking that flag before proceeding with message handling.

Fixes flutter/flutter#160003

[flutter-dashboard]

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