const -> static const

Staged at:
http://book.dart-lang.appspot.com/docs/dart-up-and-running/contents/preface.html#preface-updates
http://book.dart-lang.appspot.com/docs/dart-up-and-running/contents/ch02.html#ch02-final-const

Site update CL: https://chromiumcodereview.appspot.com/16510007/

Review URL: https://chromiumcodereview.appspot.com/15817017