This is probably related to incompatibilities between the version of the SDK you are running and the version of web_ui. If you have the most recent version of both, they work together. If you don't want to use the most recent versions, then you have to explicitly manage the versions in your pubspec.yaml file.
I'm using Dart Editor version: 0.4.0_r18915 and web_ui version: 0.4.0 and it works fine.
Try getting the most recent version of Dart Editor, remove the pubspec.lock file, and run pub install again.
Meanwhile, I will figure out how to strengthen the language in the tutorial about managing versions.
Hope this helps. mem