Just about every time I set up a new `Context.Provider`, I end up accidentally specifying a `values` prop rather than `value`.  While it's a minor error, generally I build the container in which the provider resides and commit it to the code base before I ever use it.  It's only later when I go to use it that I realize I did it again.  Since the `Context.Provider` seems pretty much useless without a `value` prop specified, I'd love it if there was a prop error if it is missing... especially if another prop is defined on the `Context.Provider` instead.
static contextType = null; 
      render() {
        return this.context.hello.world;
      }
    }
    expect(() => {
      ReactTestUtils.renderIntoDocument(<Foo />);
    }).toThrow("Cannot read property 'world' of undefined");
  });
  it('should warn when class contextType is undefined', () => {
    class Foo extends React.Component {