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### Description
<!-- Example: Error thrown when calling `appendChild` on Polymer element -->

#### Live Demo
<!-- jsBin starting point (fork and edit) -->
https://jsbin.com/luhaxab/edit
<!-- glitch.me starting point (remix and edit -- must be logged in to persist!) -->
https://glitch.com/edit/#!/polymer-repro?path=my-element.html:2:0
<!-- ...or provide your own repro URL -->

#### Steps to Reproduce
<!--
Example:

1. Create `my-element`
2. Append `my-element` to document.body
3. Create `div`.
4. Append `div` to `my-element`
-->


#### Expected Results
<!-- Example: No error is throw -->

#### Actual Results
<!-- Example: Error is thrown -->

### Browsers Affected
<!-- Check all that apply -->
- [ ] Chrome
- [ ] Firefox
- [ ] Edge
- [ ] Safari 9
- [ ] Safari 8
- [ ] IE 11

### Versions
<!--
`Polymer.version` will show the version for Polymer
`bower ls` or `npm ls` will show the version of webcomponents.js or webcomponents-lite.js
-->
- Polymer: vX.X.X
- webcomponents: vX.X.X 
