StyleAnimation: code review
helpers: animation: make element argument required
Duration always in milliseconds

1) Having consistent types exposed is worth it
2) very easy for someone w/ a duration to pass in duration.inMilliseconds
DRY'd up access to 'now' milliseconds

...will likely end up using high precision time via Performance
Removed dependency on dart:isolate:Timer

since this is web-only, window.setTimeout is fine
Merge pull request #9 from kevmoo/master

more code review