[wireframe] Spark DropDownList has different prompt truncation behavior in Spark and Wireframe themes

Found in 4.5.0.18108

Steps to reproduce:
1. Compile and run attached MXML file using the default Spark theme and Wireframe theme.

Actual Results:
Spark theme truncates the prompt text.
Wireframe theme clips the prompt text.

Expected Results:
Spark and Wireframe should have similar behaviors.

Workaround (if any):
n/a