No error for undeclared #private property in `.js` files

class Thing {
    /**
     * @param prop {string}
     */
    constructor(prop) {
        this.#prop = prop;
    }
}

This class assigns to #prop, but it’s an error to use #prop without declaring it.

This is reported as a SyntaxError, so this should report an error even outside of checkJs.

[DanielRosenwasser]

(CC @robpalme in case anyone on the team is interested)

[sandersn]

I don’t think this should be an error in JS without checkJs on. We don’t report errors for assignment to const either, which is another error that can be lexically determined as well.