Changed interfaces to abstract classes in utils. Review URL: https://chromiumcodereview.appspot.com//10915231
Changed interfaces to abstract classes in utils.
Review URL: https://chromiumcodereview.appspot.com//10915231

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@12263 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/pub,dart-lang/pub,computmaxer/pub,cjkao/dart-pub,maxwellpeterson-wf/pub