I suppose there's no harm done in leaving source=1.6 but building with java7... ant will just complain a little. Although target should at least probably be set to target=1.7 so that ant makes sure it outputs j7 code (sets the compatibility mode for the javac compiler).
that bug appears to have been fixed in jdk6 actually 
-- however there are a lot of other bugs in j6 that won't ever be fixed due to EOL, etc. Number of Swing bugs as well as underlying AWT bugs that effect the fluidness of spark.