Make inputTrees immutable about broccoli-plugin HOT 4 CLOSED

Yes, trees are definitely supposed to be immutable, and we're not enforcing that at the moment.

@stefanpenner mentioned some use cases where Ember CLI addons want to modify the set of inputTrees at initialization time (when Brocfile.js runs basically, so before the Broccoli builder kicks in). I'm still unsure - we could allow early mutation and then freeze it, or maybe we should find a different approach and really have trees be completely immutable. /cc @rwjblue

from broccoli-plugin.

Allowing early mutations would be acceptable once outputPaths are fully immutable (the other issue), but I think it still produces code which more difficult to reason about than it should be

from broccoli-plugin.

Cc @chadhietala

from broccoli-plugin.

This has gone stale, I believe our current architecture addresses many of these concerns.

from broccoli-plugin.