I'm wondering if it makes sense to try and generalize this off the bat a little.  We could use a map to map that msg-up-btn and msg-down-btn are header buttons and that the way we test for them being enabled/disabled is to run a elem.querySelector('.icon-disabled').

Like:
```
headerButtonEnabled = { selector: '.icon-disabled', nullResult: true }
enabledMap = { 'msg-up-btn': headerButtonEnabled, 'msg-down-btn': headerButtonEnabled }

function isEnabled(elem) {
  var enabledInfo = enabledMap[elem.classList[0]];
  var nullResult = (elem.querySelector(enabledInfo.selector) === null);
  return enabledInfo.nullResult === nullResult;
}
```
or we could just have a list of button classes or maybe some inference.

But basically I think it makes sense to avoid having this method do hard-coded element traversal, especially since it has such a generic name.