[SPIKE] Detect and propagate file changes on rebuild.

This should make it possible to live-reload only changed output files.

Needs review + tests.

/cc @stefanpenner 
not really, depends on the byte size of the output dir. 

Quick test for 24,009 files in a typical ember-cli apps node_modules tree, it takes 1874ms. Clearly thats far to long for incremental builds, but the output dir of most apps hopefully doesn't come anywhere near this.