update link to github
old example was removed at https://github.com/sethladd/dart-polymer-dart-examples/commit/0c79cd6c7b58285175a4446db004d95cf4a28d73
Merge pull request #304 from d2m/master

update link to github
fixed formatting of dart2js yaml

R=rnystrom@google.com, shaza@google.com

Review URL: https://codereview.chromium.org/178253003