Summary:
Can't run `RNTester` app and got the error below:

![image](https://user-images.githubusercontent.com/6890034/29361000-1ae6cfce-82b8-11e7-9522-b827de2712f7.png)

Make prospective syntax(trailing commas in function parameter lists and calls, supported in ES8) compatible.

If correct, the `RNTester` can be started.
Closes https://github.com/facebook/react-native/pull/15510

Differential Revision: D5644673

Pulled By: TheSavior

fbshipit-source-id: df34638f37b81f04cea5cf7a25fc7405d4321ea6