Support ES7: exponentiation operator

[mhegazy]

Here is the ES7 proposal: https://.com/rwaldron/exponentiation-operator
As of today, the proposal is at Stage 3.

[danquirk]

For posterity, the original issue asking us to support this: #2102

[bluong]

Is this compatible for ES5 or does this target only ES7?

[DanielRosenwasser]

It's compatible with ES3 onwards.