Clarify version on polymer.dart page

https://polymer-dot-dart-lang.appspot.com/polymer-dart/

Moved the pubspec.yaml section up, changed version from "any" to a range, and added a note about the version.

Also fixed dartbug.com/16246.

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com/238363002