dillon1000/react

[rhir] Promote conditional dependencies accessed by every cfg path

--- 

Implementation details summarized in comments. 

Overall, we want to calculate a `ReactiveDependencyTree` for every conditional 
block. If we know that conditional blocks are exhaustive (e.g. all CFG paths 
calculates a tree), we can take `intersection(depsFromEachBlock)` and add this 
to the parent Reactive + conditional scope `parentDeps = union(parentDeps, 
intersection(...))`. 

We use trees instead of individual deps here because we can still derive 
unconditional accesses. 

e.g. 

``` 

let x = {}; 

// props.a is an unconditional access here 

if (foo(other)) { 

x.a = props.a.b; 

} else { 

x.b = props.a.c; 

} 

```