doc(codelabs): update the deploy codelab

See dart-lang/deploy-codelab#2, dart-lang/deploy-codelab#4
Merge pull request #298 from kevmoo/master

Fixing a couple of nits mentioned in the session today
query -> querySelector in images + optipnged images

BUG=
R=kathyw@google.com

Review URL: https://chromiumcodereview.appspot.com/140123010
Merge pull request #296 from vicb/codelabs/update-deploy

doc(codelabs): update the deploy codelab