ember-cli shares broccoli trees across builders

[ef4]

After @krisselden fixed cleanup of the OneShot builders in #1084, it exposed the fact that we were leaking broccoli trees across the boundary between different broccoli builders via cacheKeyForTree. I fixed that in #1100.

But now I have also discovered that ember-cli itself stores a tree in module scope:

https://github.com/ember-cli/ember-cli/blob/a7c0d3dd932fe78369fd538bb2cb00c7329bd0a6/lib/broccoli/merge-trees.js#L6

This makes ember-cli unsafe to use when more than one broccoli builder can exist in the same process, as we do with OneShot.

This is the source of the test failure in #1085 in lts_3_28-engines ubuntu. I don't fully understand the sequence of events that causes the bug to trigger under that particular set of dependencies, but you can make it go away by doing any of these things:

• patch out the shared EMPTY_MERGE_TREE
• or patch out the builder cleanup in OneShot
• or set BROCCOLI_ENABLED_MEMOIZE=true which disables OneShot

So it's pretty clear that we're sharing the tree between the main builder and the OneShot builders, and that's blowing up.

[ef4]

I'm not sure exactly which direction to take this.

@thoov or @rwjblue are there known blockers to enabling BROCCOLI_ENABLED_MEMOIZE for everybody? If I just make embroider always turn that on, would I regret it?

@krisselden you have mentioned that the NerfHeimdal stuff is another reason to keep OneShot. What is the memory impact if we dropped OneShot and moved everything into one broccoli builder?