FileSystemEvent should be sealed

[brianquinlan]

This will allow developers to exhaustively pattern match on the Stream returned by FileSystemEntity.watch.

Unfortunately, none of the FileSystemEvent subclasses have public constructors so at least one package, package:watcher created it's own implementation to work around that.

So I think that we can solve this in 3 steps:

• Make the FileSystemEvent subclasses final and provide public constructors.
• Modify package:watcher (and possibly others) to use the public constructors instead of custom subclasses.
• Make FileSystemEvent sealed

[lrhn]

Making FileSystemEvent sealed is breaking, now and in the future, but making it final today is non-breaking because there is no 3.0 code out there yet. So we should make FileSystemEvent itself final now, not just its subclasses.
That won't break watcher until it upgrades to 3.0, and there should be time enough to fix it before that.
And provide the public constructors now too, so people have a place to go when migrating.

(The alternative is to seal the FileSystemEvent class itself, but keep the subclasses implementable. Not sure there is any long-term benefit to that, but it's a possibility. But that still breaks watcher because it actually does implement FileSystemEvent on its abstract superclass, even if it doesn't need to.)

[brianquinlan]

I made FileSystemEvent and its subclasses final.