Support Node 6 about yesno

[ryan-roemer]

Added another work item for engines.

[mscottx88]

@ianwsperber @ryan-roemer Can this be done just through changing the target in tsconfig.json from "ES2018" to "ES5"?

[ryan-roemer]

It technically could, but at least for babel, the generated code from async/await is just a monstrosity of awful, so really would be better to have native, terse promises. Maybe we look at the TS generated output for that and see first if it's equally as bad?

... as another factor to consider node6 is EOL'ed April 2019

[ryan-roemer]

One other thing to keep in mind, we only need to do this for src, not examples or test (we could just omit testing 6 in CI). Doing a quick search just yielded only a handful of async/await in actual src.

Easiest path may just be to swap those out, leave CI as is and be done with it!

[keithcom]

node 6 is no longer in LTS