update firebase/firebase.d.ts to version 2.0.2

Thank you for your advice.
I thought there was a better way, I read it again several guidelines.

I think "Class Decomposition" in the following document keeps the correctness of the declarations and can remove the duplicate declaration.
https://typescript.codeplex.com/WikiPage?title=writing%20Definition%20%28.d.TS%29%20Files

So I tried to fix, so please check thank.