Allow generic extended construction.

GenericClassConstructor.ts

interface FooConstructor<T> {
    new(val: number): T; 
}

interface FooInterface {
    myVal: number;
}

class Foo implements FooInterface {
    myVal: number; 

    constructor(val: number) {
        this.myVal = val;
    }

    doFoo() { }
}

class Bar<T extends FooInterface> {
  public doot: T;
  constructor(ctor: FooConstructor<T>) {
     this.doot = new ctor(4);
  }
}

const f = new Bar(Foo);