Use HashMap, HashSet from dart:collection – faster!

[kevmoo]

https://github.com/dart-lang/graphs/blob/d329d755cae8ce5c472842ec51ff2229ee8c481b/lib/src/strongly_connected_components.dart#L29

If you don't care about insertion ordering, you'll get better perf and mem usage if you use the Hash* flavor

[kevmoo]

In the case I just looked at, I can run the connected components logic ~3200 times in one second w/ Hash* over ~2700 times per second w/ LinkedHash*

[kevmoo]

FYI: this is VM, not dart2js