[fuchsia,dart:io] Emulate edge-triggering of close events

The asynchronous waiting mechanism used by the eventhandler thread
on Fuchsia is inherently level-triggered. However, since the upper
layers of the dart:io socket implementation expect edge-triggering,
the Fuchsia eventhandler has to emulate edge-triggering.

Previously, this edge-triggering was emulated for kInEvent and
kOutEvent, but not kCloseEvent. It was not thought that simulating
edge-triggering for kCloseEvent was necessary since the socket would
soon be going down, and that a few redundant messages would be
harmless. This turned out not to be the case. In practice, the
eventhandler thread may flood the Dart thread with kCloseEvent
messages.

This CL adds the edge-triggering emulation for kCloseEvent. The
Dart thread will no longer be flooded with kCloseEvent messages.

Unfortunately, the eventhandler thread may still spin on Fuchsia's
equivalent of POLLHUP signals until the Dart thread destroys the
socket, as these cannot be unsubscribed from.

Fixes b/150902962

Change-Id: I2784220197771508689504156a761842ac164c8e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143888
Reviewed-by: Zichang Guo <zichangguo@google.com>
Commit-Queue: Zach Anderson <zra@google.com>