I assume the transformation is not hinted, because e.g. animating ^ to double ^ (like chevron) would be (subjectively) better by cloning the ^ and shifting it down.
It's a beautiful image free icon set with full transition animations. It's good for putting dynamic icons into sites and apps without booting up GIMP or Photoshop.
Is there a 'Material Design' recommendation towards mutating icons? I would guess not, because the sorts of tangible things in which it finds its inspiration don't mutate in place. (They might slide in/out/over/etc.)
update: I did find a suggestion that such mutations can be 'delightful details', at http://www.google.com/design/spec/animation/delightful-detai... – seems a bit of a stretch to me, a subtle, non-'material' UI change that's quite pretty but perhaps easy-to-miss or prone-to-user-error. I suppose as long as it's confined to toggles, as in the page's examples, the flourish is low-risk.
