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 = 'foo';
      render() {
        return this.context.hello.world;
      }
    }
    expect(() => {
      expect(() => {
        ReactTestUtils.renderIntoDocument(<Foo />);
      }).toThrow("Cannot read property 'world' of undefined");
    }).toErrorDev(
      'Foo defines an invalid contextType. ' +
        'contextType should point to the Context object returned by React.createContext(). ' +
        'However, it is set to a string.',
    );
  });
  it('should warn if you define contextType on a function component', () => {
    const Context = React.createContext();
    function ComponentA() {