dillon1000/react

Mutation within a reactively controlled block propagates reactivity

In InferReactivePlaces, we already account for reactively controlled values: 
where a value is never assigned a non-reactive value, but _which_ value is 
assigned is based on a reactive condition (the test conditions of an if, switch, 
loop, etc). 

This PR extends that reactively-controlled inference to mutation that is 
conditioned upon a reactive value. From the test case: 

```javascript 

let x = []; 

if (props.cond) { 

// This mutation has no reactive inputs. 

// *But* the mutation conditionally occurs based on props.cond which is reactive 

x.push(1); 

} 

let y = false; 

if (x[0]) { // therefore the value observed here is reactive 

y = true; 

} 

// so the value of y here is reactive via the reactive control dependency x[0] 

return [y]; 

```