User DOM not found when rendering multiple <content>'s as a result of a batched "set"

[akagomez]

This is a complex and specific use case, so I had a hard time titling this issue. See the demo:

http://jsbin.com/bacuba/3/edit?html,js,console,output#J:L30-34

The example will work as expected if:

• If the Light DOM does not contain a component
• If the "set" that causes the Light DOM to be added is not batched

While researching with @justinbmeyer we were able to determine that the content property is removed here and not added back here before the next item is rendered.

When not batched, this isn't the case. The content property is unset/set before progressing to the next item in the series.

/cc @imjoshdean