Tests with `Isolate.run` pause indefinitely.

[HosseinYousefi]

This never prints 42 when it's run using --pause-isolates-on-exit.

import 'dart:isolate';

void main() async {
  print(await Isolate.run(() => 42));
}

And since coverage uses the --pause-isolates-on-exit flag (presumably to communicate with the VM service), the tests with Isolate.run time out.