Fix #4877: Exponentiation operators

[GeoffreyBooth]

Fixes #4877: Output ES2018 versions of ** and **= operators.

You can’t do odd things like !2 ** 2 or ~1 ** 5 in JavaScript, so I removed those tests. (Wrap the left side in parentheses and it’ll work.) I suppose technically this is a breaking change, but the test is really just testing order of operations and the examples seem unlikely to exist in production code.

These operators are only supported in Node 7.5+.

[GeoffreyBooth]

I’ve updated this to account for #5003. I think this should be ready now. @zdenko do you have any notes?

I’d like to resolve #5014 and any other open issues first in a 2.2.4 patch release, then merge in all the enhancements that are ready and release them in a 2.3.0. So @zdenko if you think this PR looks good, please just add a comment saying so, and we can merge it into master once 2.2.4 is done.

[zdenko]

LGTM 👍

[aminland]

Should we wait for this to be stage 4 first? otherwise you have to use the plugin transform-exponentiation-operator unless we don't care about IE anymore...

[GeoffreyBooth]

This feature is Stage 4: https://github.com/tc39/proposals/blob/master/finished-proposals.md

[GeoffreyBooth]

This feature doesn’t require a rewrite of the Compatibility section, so it’s safe to merge in as is.