I opened up the "error log" view in Eclipse, and saw that Macromedia (now Adobe)'s parser was throwing an exception "java.lang.ClassCastException: macromedia.asc.parser.MetaDataEvaluator$KeyValuePair cannot be cast to macromedia.asc.parser.MetaDataEvaluator$KeylessValue"
No help there.
I then found the issue. In my case, it was a bad coding that was actually valid syntax:
private var fileLoadIsComplete = Boolean = false;
This should be
private var fileLoadIsComplete : Boolean = false;
My only advice is to start commenting out recent code until you find the offending line that caused the "internal build error".