Right now we validate some of our proptype definitions, but we don't surface the issue until that component class is consumed. We should instead validate these, in `__DEV__` only as warnings when the class in created. This means that a call to `React.PropTypes.onOf('foo')` results in an immediate warning logged.

This is a relatively contained bit of code so would be a good candidate for somebody looking to start contributing to React.

});
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.any, null);
      typeCheckPass(PropTypes.any, undefined);
    });
    it('should warn for missing required values', function() {
      typeCheckFail(PropTypes.any.isRequired, null, requiredMessage);
      typeCheckFail(PropTypes.any.isRequired, undefined, requiredMessage);
    });
  });
  describe('ArrayOf Type', function() {
    it('should fail for invalid argument', function() {
      typeCheckFail(
        PropTypes.arrayOf({ foo: PropTypes.string }),
        { foo: 'bar' },
        'Property `testProp` of component `testComponent` has invalid PropType notation inside arrayOf.'
      );
    });
    it('should support the arrayOf propTypes', function() {
      typeCheckPass(PropTypes.arrayOf(PropTypes.number), [1, 2, 3]);
      typeCheckPass(PropTypes.arrayOf(PropTypes.string), ['a', 'b', 'c']);
      typeCheckPass(PropTypes.arrayOf(PropTypes.oneOf(['a', 'b'])), ['a', 'b']);
      typeCheckPass(PropTypes.arrayOf(PropTypes.symbol), [Symbol(), Symbol()]);
    });
    it('should support arrayOf with complex types', function() {
      typeCheckPass(
        PropTypes.arrayOf(PropTypes.shape({a: PropTypes.number.isRequired})),
        [{a: 1}, {a: 2}]
      );
      function Thing() {}