Keep a different scope in the parser for a function's parameters

[Saam Barati]

Because we don't do this, a function's default parameter values can cause us to think something is captured when it isn't.
for example:
```
let inner;
function foo(a = function() { return inner; }) {
    let inner; // We will mark this as captured even though it isn't.
    return inner;
}
```