Add "transformers" line to pubspec.yaml excerpts

Staged at:
https://1065-dot-dart-lang.appspot.com/polymer/#installing-polymerdart
https://1065-dot-dart-lang.appspot.com/polymer/using-elements/

BUG=https://github.com/dart-lang/www.dartlang.org/issues/1065
R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/613683005