add tsignore message

Author: Brian Chen

packages/firestore/test/util/node_persistence.ts

@@ -16,7 +16,7 @@
  */
 
 import * as fs from 'fs';
-// @ts-ignore
+// @ts-ignore: There are no types for indexdbshim.
 import * as registerIndexedDBShim from 'indexeddbshim';
 import * as os from 'os';