dillon1000/react

[babel][contextvar] Patch context identifier babel logic; only use referenced identifiers

--- 

A few fixes for finding context identifiers: 

Previously, we counted every babel identifier as a reference. This is 
problematic because babel counts every string symbol as an identifier. 

```js 

print(x);  // x is an identifier as expected 

obj.x      // x is.. also an identifier here 

{x: 2}     // x is also an identifier here 

``` 

This PR adds a check for `isReferencedIdentifier`. Note that only non-lval 
references pass this check 

```js 

print(x);  // isReferencedIdentifier(x) -> true 

obj.x      // isReferencedIdentifier(x) -> false 

{x: 2}     // isReferencedIdentifier(x) -> false 

x = 2      // isReferencedIdentifier(x) -> false 

``` 

Which brings us to change #2. 

Previously, we counted assignments as references due to the identifier visiting 
+ checking logic. The logic was roughly the following (from #1691) 

```js 

contextVars = intersection(reassigned, referencedByInnerFn); 

``` 

Now that assignments (lvals) and references (rvals) are tracked separately, the 
equivalent logic is this. Note that assignment to a context variable does not 
need to be modeled as a read (`console.log(x = 5)` always will evaluates and 
prints 5, regardless of the previous value of x). 

``` 

contextVars = union(reassignedByInnerFn, intersection(reassigned, 
referencedByInnerFn)) 

``` 

--- 

Note that variables that are never read do not need to be modeled as context 
variables, but this is unlikely to be a common pattern. 

```js 

function fn() { 

let x = 2; 

const inner = () => { 

x = 3; 

} 

} 

```