Java 8 Stream Collecting Set

To better understand the new stream API I'm trying to convert some old code, but I'm stuck on this one.

 public Collection<? extends File> asDestSet() {
    HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>();
    //...
    Set<File> result = new HashSet<File>();
    for (Set<File> v : map.values()) {
        result.addAll(v);
    }
    return result;
}

I can't seem to create a valid Collector for it:

 public Collection<? extends File> asDestSet() {
    HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>();
    //...
	return map.values().stream().collect(/* what? */);
}

Use flatMap:

return map.values().stream().flatMap(Set::stream).collect(Collectors.toSet());

The flatMap flattens all of your sets into single stream.