simple fix: loosen type annotations for ShadowRoot
simple fix: loosen type annotations for ShadowRoot

R=sigmund@google.com

Review URL: https://codereview.chromium.org//32903004

git-svn-id: 0aa7e0b98478d6ef7145d04e8154f8ab2209deba@28957 260f80e4-7a28-3924-810f-c04153c831b5

dam0vm3nt/polymer-dart,ronlobo/polymer-dart,dam0vm3nt/polymer-dart,jakemac53/polymer-dart,jakemac53/polymer-dart,dart-archive/polymer-dart,ronlobo/polymer-dart,meflyup/polymer-dart,meflyup/polymer-dart,dart-archive/polymer-dart