dillon1000/react

Combine dependency collection and propagation, more precise collection

I realized that properly propagating scope dependencies requires reusing the 
same logic as dependency collection itself: a dependency of an inner scope 
should only be propagated upward if the dependency was declared before the outer 
scope, for example. So this PR reimplements dependency collection in the 
propagation pass. 

At the same time I made a few other improvements: 

* Don't report dependencies that are "constant". This is a bit simplistic for 
now, we can use a more advanced analysis later. 

* Try to avoid creating duplicate dependencies. 

This addresses the todo from the previous PR (flattening scopes in loops) since 
now we don't need to compute deps until after that runs. As a follow-up i'll 
remove the existing dependency collection.