
The change record on array mutation is not using the number indicate the index, but using it as the key, so can not retrieve the item by the number, use `this.get` as recommended from the doc. https://polymer-library.polymer-project.org/1.0/docs/devguide/observers Bug: Issue 11375 Change-Id: I04da93aa7c01ec40bddc91d2400bceac101f7c62