Dart, constraints on Generics?

You can specify type constraints like this :

class StackPanel<TBase extends SomeType> extends Panel<TBase> {
}

The language specification says :

A type parameter T may be suffixed with an extends clause that specifies the upper bound for T. If no extends clause is present, the upper bound is Object. It is a static type warning if a type parameter is a supertype of its upper bound. The bounds of type variables are a form of type annotation and have no effect on execution in production mode.