Similarly to how we changed ts2dart plugin in #1720, we should do the same for broccoli-funnel.

I suggest we fork funnel, wrap it into DiffingBroccoliPlugin and have it incrementally respond to changes.

Since funnel is a third-party code and DiffingBroccoliPlugin is not on npm yet, likely the best way to do this would be to pull in the fork via npm and wrap it into the DiffingBroccoliPlugin in the angular/angular repo. That way we don't need to have 3rd party code in the angular repo and don't need to export the diffingbroccoliplugin to npm just yet (I still have some bigger changes planned before it goes on npm).

This work should take off about 200-500ms of build latency.