The class org/apache/maven/artifact/transform/SnapshotTransformation was originally (apache-maven-2.2.1) bundled in maven-2.2.1-uber.jar
However this is now missing from maven 3.0.* releases.
As a result, one of the flex-mojo's sonatype project goals breaks when run under maven 3.0.* series.
The bug report is here.
It's been marked as Fixed as the developer is adamant that API compatability is supposed to be maintained between Maven 2 and 3 series but is very much still active.
There are no more releases planned in that series but it is depended upon by many developers.