[Bug] Crashlytics plugin - keys don't work 

<img width="663" alt="Screen Shot 2019-05-17 at 11 38 12 AM" src="https://user-images.githubusercontent.com/13853/57901575-581a6e00-7898-11e9-922c-9e65dd037539.png">

I am using the latest version firebase_crashlytics: 0.0.4+3, but there is a issue in the _prepareKeys() function , could you please confirm and fix it ? thank you.

 @Hixie @kroikie 