fix a race condition with isolate notification (#4286)

* fix a race condition with isolate notification

* return a future

packages/flutter_tools/lib/src/commands/run.dart

@@ -394,10 +394,10 @@ class _RunAndStayResident {
       observatory = await Observatory.connect(result.observatoryPort);
       printTrace('Connected to observatory port: ${result.observatoryPort}.');
 
+      observatory.populateIsolateInfo();
       observatory.onExtensionEvent.listen((Event event) {
         printTrace(event.toString());
       });
-
       observatory.onIsolateEvent.listen((Event event) {
         printTrace(event.toString());
       });

packages/flutter_tools/lib/src/observatory.dart

@@ -74,6 +74,11 @@ class Observatory {
     _getEventController(data['streamId']).add(event);
   }
 
+  Future<Null> populateIsolateInfo() async {
+    // Calling this has the side effect of populating the isolate information.
+    await waitFirstIsolate;
+  }
+
   Future<IsolateRef> get waitFirstIsolate async {
     if (isolates.isNotEmpty)
       return isolates.first;