Incompatibility with Flutter >1.9.0

[jmagman]

cocoapods-integrate-flutter will no longer with with any version of Flutter greater than or equal to 1.9.0. In particular, podhelper.rb no longer uses Generated.xcconfig and that file should never be #included anywhere.

New instructions:
https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps#add-your-flutter-app-to-your-podfile
Migration instructions:
https://github.com/flutter/flutter/wiki/Upgrading-Flutter-added-to-existing-iOS-Xcode-project
Flutter PR with most of the changes:
flutter/flutter#36793

podhelper template:
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/templates/module/ios/library/Flutter.tmpl/podhelper.rb.tmpl

Usage:
https://github.com/flutter/flutter/blob/master/dev/integration_tests/ios_host_app/Podfile