This-type fix for prototypes only works at top-level

[sandersn]

#26925 only works at top-level:

(function () {
    function Brunch(children) {
        this.parent = null;
    }

    Brunch.prototype = {
        removeInner: function(at, n) {
            this.collapse();
        },

        collapse: function() {
        },
    };
});

Expected behavior:
No error on this.collapse and this: Brunch & { removeInner, collapse }

Actual behavior:
Error on this.collapse and this: Brunch.

[sandersn]

The better typing in #26295 exposes a bug in the binder: the node that is used to look for the containing class of prototype assignments was incorrect.