[io] Rewrite Mac OS X FS watcher

Watcher used pipes to fit into the "socket"-like implementation used by
other OSes. However it turns out that pipe buffer on Mac OS X is not
necessarily large enough to fit the whole FSEvent structure meaning that
we can't expect to be able to read FSEvents atomically from a pipe.

This situation happens because Mac OS X inherits BSD behavior of
limiting total number of kernel memory reserved for pipe buffers. Once
kern.ipc.maxpipekva limit is crossed pipe buffers no longer grow to 64KB
and remain 512 bytes large (which is absolute minimum allowed by POSIX
which requires writes smaller than PIPE_BUF to be atomic).

So our code needs to be prepared that only half of FSEvent structure
(which is 1032 bytes large) will fit into pipe buffer.

We could fix this by reading each FSEvent in chunks but this seems
ridiculously ineffecient. Instead we rewrite the code to pass file
system events via SendPort instead.

Fixes https://github.com/dart-lang/sdk/issues/61551

TEST=runtime/tests/vm/dart/regress_61551_test.dart

CoreLibraryReviewExempt: VM only changes.
Change-Id: I6a6a696499044832cfb61998bc6a519bbaadf8bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451260
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>