Add a Watcher interface.

Unlike DirectoryWatcher, this interface isn't directory-specific. This
also allows ResubscribableDirectoryWatcher to become
ResubscribableWatcher, which will make it easier to implement
single-file watching.

See dart-lang/watcher#17

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1180233003.