Chained assignment breaks jsdoc type lookup

[sandersn]

// @noEmit: true
// @allowJs: true
// @checkJs: true
// @Filename: a.js

function A () {
    this.x = 1
}
/** @param {number} n */
A.prototype.y = A.prototype.z = function (n) {
    return n + this.x
}
var a = new A()

Expected behavior:
a.y : (n: number) => number and a.z: (n: number) => number.

Actual behavior:
a.y : (n: any) => any and a.z: (n: any) => any.

Note that binding works fine; it's just jsdoc type lookup that is defeated by chained assignment. Chained assignment of statics breaks in the same way as prototype assignments.