Support for --target ES2018

[mhegazy]

As TC39 gets ready to finalize ES2018, we should allow pass through for some transformation now only enabled under --target ESNext, here is the list:

Disable transforms for:

• Rest/Spread Properties
• async generators/for await ... of

We also need some new lib definitions:

• Promise finally (feat(es2018): add definition for Promise.prototype.finally #20411)

The support for --target ES2018 was added in #20385

[mhegazy]

// CC: @bterlson

[mhegazy]

// CC: @benbraou

[bterlson]

Just keep in mind that until a proposal shows up here it's not on the train for 2018.

[dinoboff]

Promise.prototype.finally, rest/spread properties and async iterable made it. Optional catch binding didn't.