code: function handleFileCloseAll(commandData) {
          return _closeList(MainViewManager.getAllOpenFiles(),
                                      (commandData && commandData.promptOnly), (commandData && commandData._forceClose));
      }
docs: Closes all open files; equivalent to calling handleFileClose() for each document, except
  that unsaved changes are confirmed once, in bulk.
  @param {?{promptOnly: boolean, _forceClose: boolean}}
  If promptOnly is true, only displays the relevant confirmation UI and does NOT
  actually close any documents. This is useful when chaining close-all together with
  other user prompts that may be cancelable.
  If _forceClose is true, forces the files to close with no confirmation even if dirty.
  Should only be used for unit test cleanup.
  @return {$.Promise} a promise that is resolved when all files are closed