Example with flutter_test

Hello @matthewshirley, 

I'm having trouble to setup pact for API integration tests. 
I have seen your example, but I'm not sure how it should work with the `flutter_test` package, if it was thought at all. 

If you already have used it in a Flutter app can you please share a simple test with two or more API calls that writes to a single pact file? 

Thank you!