Instead, we can just use the scope type to determine if a scope is a
function scope.
This fixes using `this` for parameter default values in arrow functions
crashing. This happened by `uses_this_from_environment` was not set in
`set_uses_this`, as it didn't think it was in a function scope whilst
parsing parameters.
Fixes closing modal dialogs causing a crash on https://www.ikea.com/
No test262 diff.
Reverts the functional part of 08cfd5f, because it was a workaround for
this issue.
