[Bug] `this` isn't handled when adding methods to prototypes

[SarahIsWeird]

Basically what it says on the tin. This also affects #247, when it is merged. This is a very niche thing, but it would be cool to handle. The issue is that it is quite difficult to handle, as it can happen anywhere.

Presumably fixing this would involve adding extends Object to every class declaration. However, I'm unsure if there are some built-in classes that could be extended this way, i.e. if Hackmud freezes all classes, so that needs some investigation. Because in that case, we can't do the super.valueOf() trick, if the class doesn't have some superclass.

Repro script

class Foo {
    a() {
        this.b = 1
    }
}

Foo.prototype.getB = function() {
    return this.b
}

const bar = {
    a() {
        this.b = 1
    }
}

bar.getB = function() {
    return this.b
}

[SarahIsWeird]

Another solution would be to just forbid this kind of thing if it's not resolvable.