This gives an NPE:

TreeScanner<Void,Trees> dependencyScanner = new DependencyScanner();
Trees treeUtil = Trees.instance(task);
for (CompilationUnitTree unit : trees) {
dependencyScanner.scan(unit, treeUtil);
}

This works:

TreePathScanner<Void,Trees> dependencyScanner = new DependencyScanner();
Trees treeUtil = Trees.instance(task);
for (CompilationUnitTree unit : trees) {
dependencyScanner.scan(unit, treeUtil);
}

Overloading is bad ;-)