Skip checking the source for synthetic attributes to prevent warning.

[navels]

This fixes a warning caused when template parsing encounters a synthetic attribute (i.e., inserted by a plugin). For example, building the ember-maybe-in-element addon outputs these warnings:

unexpectedly found "<div id=\"original-p" when slicing source, but expected "@destinationElement"
unexpectedly found "<div id=\"origi" when slicing source, but expected "@renderInPlace"
unexpectedly found "    \n      <div id=" when slicing source, but expected "@destinationElement"
unexpectedly found "    \n      <di" when slicing source, but expected "@renderInPlace"

and the same will happen for any app that uses the ember-basic-dropdown addon (which uses the above addon).

These should fix the remaining causes of emberjs/ember.js#19392 that are not already fixed by #1430.

I tried to add a test that would be even vaguely useful and couldn't really come up with anything.