message: test: empty Observable.toPromise
  show that an empty Observable results in a promise resolved with a
  value of undefined
diff: @@ -12,6 +12,13 @@ describe('Observable.prototype.toPromise', () => {
  });
  });
  + it('should convert an empty Observable to a promise of undefined', (done: MochaDone) => {
  + Observable.empty().toPromise(Promise).then((x) => {
  + expect(x).to.be.undefined;
  + done();
  + });
  + });
  +
  it('should handle errors properly', (done: MochaDone) => {
  Observable.throw('bad').toPromise(Promise).then(() => {
  done(new Error('should not be called'));
  