<javac> fails with NPE when compiling with eclipse ecj 3.1.x
Bugzilla 40839.
root cause of the problem was in org.eclipse.jdt.core.JDTCompiler
method addExtDirs. A FileSet was created without the Project attribute set,
then added to a Path.
git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@469050 13f79535-47bb-0310-9956-ffa450edef68