`flatMap` should close both iterators when `return`/`throw` are called, not just the outer one

[bakkot]

Consider:

function* words() {
  try {
    yield 'abc';
    yield 'xyz';
  } finally {
    console.log('underlying iterator closed');
  }
}

let chars = words().flatMap(function* (arg) {
  try {
    yield* arg.split('');
  } finally {
    console.log('mapping iterator closed');
  }
});

for (let c of chars) {
  break;
}

I would expect this to print 'mapping iterator closed' as well as 'underlying iterator closed'. But it doesn't; only the underlying one gets closed.

This is related to #114, but more so: even the JS which that issue says is roughly equivalent to the current implementation would close the mapping iterator.

[hax]

I found that Firefox nightly actually work as your expect, though core-js implementation not.