code: function _handleCurrentFileChange(e, file) {
          var doc = file && DocumentManager.getOpenDocumentForPath(file.fullPath);
          _notifyActiveEditorChanged(doc && doc._masterEditor);
      }
docs: Current File Changed handler
  MainViewManager dispatches a "currentFileChange" event whenever the currently viewed
  file changes.  Which could mean that the previously viewed file has been closed or a
  non-editor view (image) has been given focus.  _notifyAcitveEditorChanged is also hooked
  up to editor.focus to handle focus events for editors which handles changing focus between
  two editors but, because editormanager maintains  a "_lastFocusedEditor" state, we have to
  "nullify" that state whenever the focus goes to a non-editor or when the current editor is closed
  @private
  @param {!jQuery.Event} e - event
  @param {?File} file - current file (can be null)