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.

function Engineer() {}
      Engineer.prototype = new Person();
      typeCheckPass(PropTypes.instanceOf(Person), new Person());
      typeCheckPass(PropTypes.instanceOf(Person), new Engineer());
      typeCheckPass(PropTypes.instanceOf(Date), new Date());
      typeCheckPass(PropTypes.instanceOf(RegExp), /please/);
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.instanceOf(String), null);
      typeCheckPass(PropTypes.instanceOf(String), undefined);
    });
    it('should warn for missing required values', function() {
      typeCheckFail(
        PropTypes.instanceOf(String).isRequired, null, requiredMessage
      );
      typeCheckFail(
        PropTypes.instanceOf(String).isRequired, undefined, requiredMessage
      );
    });
  });
  describe('React Component Types', function() {
    beforeEach(function() {
      MyComponent = React.createClass({
        render: function() {