Conform to the latest breaking changes in Dartium and fixes in Polymer Dart

1) Single *.dart script tag per HTML is required by Dartium now.
2) Polymer now allows only 1 main() per app, and that used to be taken by init.dart.
3) However, Polymer also fixed an bug around that, and now <script src='init.dart'> can be replaced with initPolymer() in the app's own main().