I just made a similar test in 1.0.0.3_r30187 and I don't get this error. If you don't use the last stable version of Dart you should update your version.
Here's my tested code :
import 'dart:html';
import 'dart:mirrors';
class Component {
Element element ;
Component(this.element) {
this.element.children.add(new Element.html("<b>Dart rocks</b>"));
}
}
class ButtonComponent extends Component {
ButtonComponent(element) : super(element);
}
main() {
final a = reflectClass(ButtonComponent).newInstance(new Symbol(''),
[document.documentElement]).reflectee;
print(a); // display : Instance of 'ButtonComponent'
}