def collection = newUnionFileCollection(source1, source2)
        collection.files.toList() == [file1, file2, file3]
        collection.sourceCollections == [source1, source2]
    }

    def contentsTrackContentsOfSourceCollections() {
        def source1 = Mock(FileCollectionInternal)
        def source2 = Mock(FileCollectionInternal)

        def collection = newUnionFileCollection(source1, source2)

        when: