Fix a race in runFromSource. (#9495)

There is a race where we could complete a future based on a stream event
and then before we cancel the stream subscription, we get another event
and try and complete the future again.

packages/flutter_tools/lib/src/vmservice.dart

@@ -1124,6 +1124,7 @@ class FlutterView extends ServiceObject {
       // launch errors.
       if (event.kind == ServiceEvent.kIsolateRunnable) {
         printTrace('Isolate is runnable.');
+        if (!completer.isCompleted)
           completer.complete(null);
       }
     });