dillon1000/react

Add test case for invalid lambdas

This is the example we discussed in our design sync. 

```javascript 

function Component(props) { 

const [x, setX] = useState({ value: "" }); 

const onChange = (e) => { 

// INVALID! should use copy-on-write and pass the new value 

x.value = e.target.value; 

setX(x); 

}; 

return <input value={x.value} onChange={onChange} />; 

} 

``` 

Here `onChange` is a mutable lambda, and it should be invalid to pass a mutable 
lambda where a frozen value is expected. This is because unlike other value 
types, you cannot freeze a lambda — the only choice is to not call it at all. 

Note that there is a harder case to catch: 

```js 

function Component(props) { 

const [x, setX] = useState({ value: "" }); 

const onChange = (e) => { 

// INVALID! should use copy-on-write and pass the new value 

x.value = e.target.value; 

setX(x); 

}; 

const x = constructAValueThatMaybeAliasesItsInput(onChange); 

return <input value={x.value} onChange={x.maybeGetTheLambdaBack()} />; 

} 

``` 

This case demonstrates how mutable lambdas can be captured and then accessed 
later — the analysis to catch this case is more sophisticated bc it involves 
inferring that `x` aliases a mutable lambda. But we also can't be sure that `x` 
does alias the lambda, so disallowing this code could prevent a lot of valid 
code from compiling. My hypothesis is that we should start with at least 
validating the example at the top, while allowing the second case for now.