Check for existence of `transitions` class on tree to determine if we're animating.
Check for existence of `transitions` class on tree to determine if we're
animating.

The previous check wasn't always accurate. If the `_transitionWrap`
function determined there were too many nodes to transition, we wouldn't
use transitions, but the `if` statement would think we are
transitioning.

SpiderStrategies/kalpa-tree,SpiderStrategies/kalpa-tree