function* foo() {
  var a = yield wat(),
      b = 2;
  var c = yield a = b;
  yield a, yield b;
  yield a = b;
  return yield 1 || yield 2;
}

function* foo() {
  var a = yield wat(),
      b = 2;
  var c = yield a = b;
  yield a, yield b;
  yield a = b;
  return (yield 1) || (yield 2);
}

Fix failing test