FR: Bazel support

### [REQUIRED] Describe your environment

* Operating System version: macOS Mojave (10.14.4)
* Browser version: Chrome 70-something
* Firebase SDK version: Latest
* Firebase Product: All of them

### [REQUIRED] Describe the problem

We have a frontend project using Bazel, specifically `rules_closure`. It would be awesome to gain Bazel support in the Firebase JS SDK, so we could potentially build against the SDK's sources.

The gains delivered by this support might include a deeper ability to tree-shake unused code from the Firebase SDK, for a given application.

#### Steps to reproduce:
1) Try to use Firebase SDK in JS from Bazel with `rules_closure`
2) Realize you will probably have to write an overlay file and use `new_git_repository`, or find a fork, or build against externs
3) Externs are not ideal because they are out-of-scope of the Closure compiler

Thank you in advance Firebase team