moved from spans to col-md and upgraded h4 and h5 headers in codelab

BUG=
R=kathyw@google.com

Review URL: https://chromiumcodereview.appspot.com/65493005
removing line numbers from code samples

BUG=
R=kathyw@google.com

Review URL: https://chromiumcodereview.appspot.com/71883002
Merge branch 'dart-1.0' of https://github.com/dart-lang/dartlang.org into dart-1.0