SDK update

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Add feature to detect visibility
Fix visibility detection
Merge pull request #21 from pjako/feature_visibility

Feature to detect visibility
A gameloop that runs without dart:html or dart:io
Merge pull request #23 from pjako/isolate_game_loop

A gameloop that runs without dart:html or dart:io
Split into GameLoopIo and GameLoopHtml.

Signed-off-by: John McCutchan <john@johnmccutchan.com>
GameLoopIo->GameLoopIsolate.

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Started implementing periodic
Adding periodic implementation
Periodic test and addTimer named parameter.
making _isDead() property