Web UI applications need to be "built" (usually by a script called build.dart
in the project root, see this article for more details).
If I go to the Dart Editor, create a new test project using the wizard and selecting the Web Project (using the web_ui library) option, this creates the boilerplate including the build script.
Open up the html file and paste in the code from the github tutorial, replacing what is there already. When you save the file, the build.dart
will be invoked, outputting the build version to /web/out/
Hit the run button, and the Dart Editor will open the app in Dartium (it knows to add /out/
to the URL).