MacOS watcher can omit directory create events

[davidmorgan]

@mraleph Linux+Windows watcher are looking good now, found one more issue to dig into / work around on MacOS, with any luck the last for a while :)

Repro

import 'dart:io';

void main () async {
  final t = Directory.systemTemp.createTempSync();
  t.watch(recursive: true).listen(print);

  Directory('${t.path}/a').createSync();
  File('${t.path}/a/1').createSync();
  Directory('${t.path}/a').renameSync('${t.path}/b');
  File('${t.path}/b/1').writeAsStringSync('_');
  Directory('${t.path}/b').renameSync('${t.path}/c');
  File('${t.path}/c/1').writeAsStringSync('__');
  Directory('${t.path}/c').deleteSync(recursive: true);

  await Future.delayed(Duration(seconds: 1));
}

prints

FileSystemCreateEvent('.../a/1', isDirectory=false)
FileSystemCreateEvent('.../a', isDirectory=true)
FileSystemDeleteEvent('.../a')
FileSystemModifyEvent('.../b/1', isDirectory=false, contentChanged=true)
FileSystemDeleteEvent('.../b')
FileSystemModifyEvent('.../c/1', isDirectory=false, contentChanged=true)
FileSystemDeleteEvent('.../c/1')
FileSystemDeleteEvent('.../c')

which is giving "delete" events for directories without any corresponding "create" event.

watcher needs to figure out that a "delete" event refers to a directory, the create event would have helped :) ... e.g. when "b" is deleted we need to know it's a directory to know that "b/1" is deleted.

Workaround should be to infer the existence of "b" from the existence of "b/1" instead of relying on the directory create event for "b".