Hot Reload causes server to not exit on first ctrl-c

[jakemac53]

I noticed when playing around with --hot-reload today that it is currently causing the server process to not exit when you send the first terminate signal.

Generally this is a sign that there are some stream listeners or web socket connections still open. These are typically cleaned up here https://github.com/dart-lang/build/blob/master/build_runner/lib/src/generate/watch_impl.dart#L307, so we probably need to add some more things to that.