Flex support in the latest builds (#9618 and #9647) is completely screwed due to wrong MXML compiler invocation and/or faulty compiler configuration settings. I spent a lot of time analyzing the problem and made detailed suggestions what needs to be fixed in this JIRA ticket http://www.jetbrains.net/jira/browse/IDEA-21466. I was hoping this would be fixed in the next EAP which just came out today (#9647). For me this bug is blocker and I am stuck with #9572 which is the only build handling Flex compilation correctly. Is anything wrong with my English?