abstract class Base {
    abstract getThing(): string;
    getOtherThing() { return 'hello'; }
}
var x = new Base(); // Error, 'Base' is abstract

// Error, must either be 'abstract' or implement concrete 'getThing'
class Derived1 extends Base { }

class Derived2 extends Base {
    getThing() { return 'hello'; }
    foo() { super.getThing(); } // Error: cannot invoke abstract members through 'super'
}
var x = new Derived2(); // OK
var y: Base = new Derived2(); // Also OK
y.getThing(); // OK
y.getOtherThing(); // OK

abstract class Empty { } // OK