Invalidating project every half-second

[amcasey]

The Typings Installer polls the (initially) non-existent bower_components folder every half-second to see if it has been created. Sometime after 61a2b4a, we started receiving file change events for each poll. Here we decide that the file has been deleted (presumably, since we don't expect to receive an event for continued non-existence) and generate an Invalidate event here, which triggers a project refresh. Obviously, refreshing the project this frequently is undesirable.

[amcasey]

FYI @mhegazy @sheetalkamat

[amcasey]

Repro:

1. Enable TS server logging
2. In VS, create a JS Node.js console app
3. Watch the TI log fill up with Invalidate events

[amcasey]

Anecdotally, the issue does not appear to repro in master with #21243 reverted.