There is a guide on how to create Dart web-ui custom element in Dart code. There is also sample code for this technique
Is there any example on how to create a Dart
I found a neat little snippet they're using on the source code of chromedeveditor, e.g. here on Github
They use
factory ExampleUi() => new Element.tag('ExampleUi');
so that you could construct the element with:
new ExampleUi();