rendering problems when toggling emphasized property on Spark Button w/ errorString

Found in 4.5.0.16171

Steps to reproduce:
1. Compile and run attched MXML file.
2. Toggle the checkbox a few dozen times.

Actual Results:
Neat pattern, but yeah, that shouldnt happen.

Expected Results:
The emphasized skin should toggle, but you shouldnt get the stray red rendering artifacts.

Workaround (if any):
Set the Boolean emphasized prop to false, then set the error string (see workaround in test case, i also had to call validateNow())