Consider adding an informative note about the immutability of the arguments binding in strict functions

[ajklein]

In step 22.c of https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation, the spec goes out of its way to call CreateImmutableBinding when creating the binding for the arguments object for a strict function. Given that, in strict mode, any assignment to arguments is an early error, the fact that the binding is immutable is completely unobservable (so would it being mutable, for that matter).

I think it might be useful to, at the least, have a NOTE here that there will never be an attempt to mutate this binding. I could also imagine removing the if statement entirely and always using CreateMutableBinding, along with the same NOTE about strict mode.