In JS, aliasing of C.prototype should cause property assignments to add to the instance

// from acorn
function Parser() {
    /** @type {number} */
    this.x = 1
}
var pp = Parser.prototype
pp.m = function () {
    this.x = "should be an error"
    return this.x
}
new Parser().m

Expected behavior:
pp.m = function() ... should add a method to the Parser instance. This should result in:

1. Error: can't assign string to property x of type number
2. No error on new Parser().m

Actual behavior:
No error on this.x = "should be an error" and an error on new Parser().m

Start by reproducing the provided JavaScript/TypeScript example and trace how the checker handles the aliased Parser.prototype, assignments to this.x, and new Parser().m. Done means the example reports the string assignment as an error while accepting the method access on the new instance.