JS: Allows illegal assignment to function properties

TypeScript Version: 3.1.0-dev.20180810

Code

const f = () => 0;
f.call = 0;
useF(f);

/** @param {() => number} f */
function useF(f) {
    f.call(null);
}

Expected behavior:

Error on assigning f.call since that would change the type.
On this topic, f.length should be readonly.

Actual behavior:

No error (at compile time).

[sandersn]

This is not a JS problem; the same hole exists in TS:

declare const f: { () => number, length: string, call: number };
function useF(f: () => number) {
  f.call(f.length)
}

I opened #29449 to track that hole, although it's been there forever, so I don't know if it's worth fixing.