Switch web-ui to use content shell

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//16578002
Fixes in WebComponent to match the latest dart:html in 0.5.14.3

Due to bug http://dartbug.com/11154, we didn't detect this on the last roll.

Review URL: https://chromiumcodereview.appspot.com//16677004
Fix tests again for 0.5.15 (TBR)

Review URL: https://chromiumcodereview.appspot.com//16673005
Merge branch 'master' of https://github.com/dart-lang/web-ui into auto_generation
Fixed comments