Generics: Restricting the parameterized type

class Foo<T extends SomeBaseClass> { 
   // Implementation goes here... 
   String toString() => "Instance of 'Foo<$T>'"; 
} 

class Extender extends SomeBaseClass {...}

var someBaseClassFoo = Foo<SomeBaseClass>(); 
var extenderFoo = Foo<Extender>();

var foo = Foo(); 
print(foo); // Instance of 'Foo<SomeBaseClass>'