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

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index e9df65d..332f352 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -1124,7 +1124,8 @@
       // launch errors.
       if (event.kind == ServiceEvent.kIsolateRunnable) {
         printTrace('Isolate is runnable.');
-        completer.complete(null);
+        if (!completer.isCompleted)
+          completer.complete(null);
       }
     });
     await owner.vm.runInView(viewId,