Dart confusing TYPE error
This message looks like web_ui is failing a type check.
The similar SO question MarioP links to has a stacktrace showing this message coming from within the web ui library itself.
Do you have an xmlns attribute in your html?Have a look at this web-ui issue.
AttributeName is defined within the html5lib library. Perhaps web-ui is expecting a newer version of this library but getting an older one. Make sure you've done a pub install. You can also try deleting your packages folder and doing a fresh pub install.
This could be caused by out-of-date packages, but it could also be a bug in web-ui. See if you can get a stacktrace and file a bug report, or post it on the web-ui mailing list.