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

#### Live Demo
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https://jsbin.com/xukike/edit?html,output

#### Steps to Reproduce
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Example:

1. Create `my-element`
2. Append `my-element to document.body
3. Create `div`.
4. Append `div` to `my-element`
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#### 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
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- Polymer: vX.X.X
- webcomponents: vX.X.X