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

```ts // 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`