fixes #459 -- add with Observable given an interface

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com//14466010
Fix breaking change from next SDK

R=dgrove@google.com

Review URL: https://chromiumcodereview.appspot.com//14910007
hot fix for bug in exit code

R=dgrove@google.com

Review URL: https://chromiumcodereview.appspot.com//14859011
(fix #224) Replace the _root field with 'getShadowRoot(tagName)'. This will get
rid of any warnings in TodoMVC.

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//14799004
Merge branch 'master' of https://github.com/dart-lang/web-ui into custom_pseudo
ready or CL