If you are compiling generators with --target ES5 --lib esnext the types of those generators will also include the iterator helpers methods like map, filter, etc., though our down-level generator emit will not have those methods even when running with newer editions of V8 that do support them:

// @target: es5
// @lib: esnext

const ar = [1, 2, 3, 4];
ar.values().map(i => i * 2); // [2, 4, 6, 8]

function * f() {
  yield * [1, 2, 3, 4];
}
f().map(i => i * 2); // error: 'f(...).map' is not a function

Even when not running in latest V8, iterator helpers can be polyfilled, e.g.:

if (typeof Iterator === "undefined") {
    let Iterator = function () {};
    Iterator.prototype = Object.getPrototypeOf(Object.getPrototypeOf([].values()));
    Iterator.prototype.map = ...;
    Iterator.prototype.filter = ...;
    globalThis.Iterator = Iterator;
}

To better support both scenarios, this makes a small change to our __generator helper to use the global Iterator.prototype object as the prototype for down-level generators, if it is present.

This also does the same for our __asyncGenerator helper when a global AsyncIterator.prototype object is present, even though async iterator helpers are still at Stage 2. Support for AsyncIterator.prototype seems harmless enough to add at this time but can be postponed to a later PR if necessary.

Please note that this will also require the same change in tslib.

Fixes #59513