Adding API documentation downloads to the archive
Adding some copy for how to use the dartdoc-viewer
update error messages for polymer

Review URL: https://chromiumcodereview.appspot.com//657903003
Updating the Polymer tutorial examples to 0.15.1.

Merge branch 'master' into polymerUpdate

Updating examples.

Updating Polymer.dart text and examples to 0.15.1.

BUG=
R=kathyw@google.com

Review URL: https://codereview.appspot.com/158990043
Addressing Kathy's comments
Merge pull request #1076 from srawlins/issue-933-api-downloads

Adding API documentation downloads to the archive
Merge branch 'master' into book2
incorporate shams's review comments 
remove contents/ directory, and add redirects
interim
done with redirects?