This disables most runtime features of dart:isolate, similar to dart:io.
For now, this supports `new ReceivePort()` and `ReceivePort.close` as
those are used by async tests to keep the Dart VM alive. Those are
no-ops in DDC. Also removes most of dart:_isolate_helper, except for
the JS API that's used to bootstrap DDC `main()`.
Change-Id: I84eab6cd39d05a70a6afe982e952076ddaaa8c0f
Reviewed-on: https://dart-review.googlesource.com/45754
Commit-Queue: Jenny Messerly <jmesserly@google.com>
Reviewed-by: Vijay Menon <vsm@google.com>
Jenny Messerly
c9e9c1d8e9