function _handleCurrentFileChange(e, file) {
        var doc = file && DocumentManager.getOpenDocumentForPath(file.fullPath);
        _notifyActiveEditorChanged(doc && doc._masterEditor);
    }
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)