[SparkStyles] Halo/MX Form doesnt codehint backgroundColor, even though it is a valid style in Halo/Spark theme

Found in 10461.

Steps to reproduce:
1. Compile and run attached MXML file (it works)
2. Remove the "backgroundColor" styles from the Form and FormItem declarations.
3. Try and get Builder to code hint the now-gone backgroundColor styles for Form and FormItem.

Actual Results:
The styles work, but arent hinted by Builder.
Not sure if this is a metadata issue or something w/ Builder, or something far more sinister.

Expected Results:
Form and FormItem should both hint backgroundColor, which works and is a valid Spark style based on the latest LangRef.

Workaround (if any):
Ignore the code hints, type it manually.