[driver] wait for the isolate to enter pause on start

packages/flutter_driver/lib/src/driver.dart

@@ -43,7 +43,21 @@ class FlutterDriver {
     VMServiceClient client = await vmServiceConnectFunction(dartVmServiceUrl);
     VM vm = await client.getVM();
     _log.trace('Looking for the isolate');
-    VMIsolate isolate = await vm.isolates.first.load();
+    VMIsolate isolate = await vm.isolates.first.loadRunnable();
+
+    // TODO(yjbanov): for a very brief moment the isolate could report that it
+    // is resumed, right before it goes into "paused on start" state. There's no
+    // robust way to deal with it other than waiting and querying for the
+    // isolate data again. 300 millis should be sufficient as the isolate is in
+    // the runnable state (i.e. it loaded and parsed all the Dart code) and
+    // going from here to the `main()` method should be trivial.
+    //
+    // See: https://github.com/dart-lang/sdk/issues/25902
+    if (isolate.pauseEvent is VMResumeEvent) {
+      await new Future.delayed(new Duration(milliseconds: 300));
+      isolate = await vm.isolates.first.loadRunnable();
+    }
+
     FlutterDriver driver = new FlutterDriver.connectedTo(client, isolate);
 
     // Attempts to resume the isolate, but does not crash if it fails because

packages/flutter_driver/test/flutter_driver_test.dart

@@ -33,7 +33,7 @@ main() {
       mockIsolate = new MockIsolate();
       when(mockClient.getVM()).thenReturn(mockVM);
       when(mockVM.isolates).thenReturn([mockIsolate]);
-      when(mockIsolate.load()).thenReturn(mockIsolate);
+      when(mockIsolate.loadRunnable()).thenReturn(mockIsolate);
       when(mockIsolate.invokeExtension(any, any))
           .thenReturn(new Future.value({'status': 'ok'}));
       vmServiceConnectFunction = (_) => new Future.value(mockClient);