libdot/hterm/nassh: make ES2018 requirement official
libdot/hterm/nassh: make ES2018 requirement official

We've been using ES2018 syntax in our linters for a while, and had
syntax like "async" in libdot deps, so we've probably been forcing
it on people without noticing.  Lets just go all in and allow all
ES2018 features as this gets us async/await & Atomics (2017), and
object rest & spread properties (2018).

Change-Id: I6cf56b4d9e2828f768be05fefc2ddb962109b01c
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2244514
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm