Also... doesn't this still need to be done for Editors that _aren't_ visible also?  And secondary inline editors?  Otherwise, when will their mode get corrected?  (Seems like a bug in the old EditorManager-based code too -- although in the current UI, you can't rename files other than the current one that's a dangerous assumption and the code still breaks if any inlines are open for the same file).

Maybe it's best to do some partial cleanup now -- e.g. have Document dispatch a "languageChanged" event and have every Editor listen to its Document and update its own mode accordingly.  (And JSLint et al could be treated as a lower-priority bug, since I think stuff like that wan't handled by the old code either).