Changing ios/Podfile ignored on re-run, pod install not invoked

## Steps to Reproduce

With single iOS device or Simulator present:

* `flutter create hello; cd hello`
* add plugin dependency to `pubspec.yaml`
* `flutter run`
* edit `ios/Podfile` arbitrarily, e.g. make it invalid
* `flutter run`
* Note: `pod install` was not re-run