App hangs when keyboard is opened on iOS 9

I'm having an issue with the app hanging when using the keyboard on iOS 9 (I don't get an issue on v11 or v12). This is with master of `flutter_webview_plugin` and Flutter v0.11.3.

# Steps to reproduce
1. Open a web view containing a text input on iOS 9 (I used iPhone 6S simulator on v9.0)
2. Focus the text input to bring the keyboard up
3. Start entering characters
4. App very quickly hangs and becomes unresponsive

The only error I get is:
```
)}; level 0.000000; active NO [wants NO]; suppression 0>] could not take process assertion
```