Add Chinese mirror sites.

Because of the Great
Firewall(http://en.wikipedia.org/wiki/Golden_Shield_Project), sometimes
can not access the GAE and Google Storage at Chinese Mainland. So we
build some mirror sites in China and translate some materials to Chinese
, let the Chinese much easier to learn Dart and use Pub. 
(Most of the time in China are unable to access 
https://commondatastorage.googleapis.com.)