fix scrollToItem in checked mode and add option to example for it
Merge pull request #145 from dart-lang/core-list

fix scrollToItem in checked mode and add option to example for it
Cherry pick https://github.com/Polymer/core-overlay/pull/33
Merge pull request #147 from dart-lang/core-overlay

Cherry pick https://github.com/Polymer/core-overlay/pull/33
Merge branch 'master' of github.com:dart-lang/core-elements into core-list-dart-test
#144
Fixes according to review comments