Use more const Widgets

[jonahwilliams]

I've noticed there are a lot of places where new can be replaced by const with just some small changes.
Every place you can use a const widget instead of new, you save flutter a little bit of work.

For example, in event_backdrop_photo.dart

  Widget _buildShadowInset(BuildContext context) {
    return new Positioned(
      bottom: -8.0,
      child: new DecoratedBox(
        decoration: *const* BoxDecoration(
          boxShadow: *const* <BoxShadow>[
            *const* BoxShadow(
              color: Colors.black38,
              blurRadius: 5.0,
              spreadRadius: 3.0,
            ),
          ],
        ),
        child: new SizedBox(
          width: MediaQuery.of(context).size.width,
          height: 10.0,
        ),
      ),
    );
  }

They aren't huge changes, but if you're interested I can open a PR

[roughike]

Sure! Sometimes it's hard to notice those places where a const can be used for me. Would love to have a PR!