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.

PropTypes.symbol,
        {
          '@@toStringTag': 'Katana',
        },
        'Invalid prop `testProp` of type `object` supplied to ' +
        '`testComponent`, expected `symbol`.'
      );
    });
    it('should not warn for a polyfilled Symbol', function() {
      var CoreSymbol = require('core-js/library/es6/symbol');
      typeCheckPass(PropTypes.symbol, CoreSymbol('core-js'));
    });
  });
  describe('Custom validator', function() {
    beforeEach(function() {
      jest.resetModuleRegistry();
    });
    it('should have been called with the right params', function() {
      var spy = jasmine.createSpy();
      Component = React.createClass({
        propTypes: {num: spy},
        render: function() {
          return <div />;
        },
      });
      var instance = <Component num={5} />;
      instance = ReactTestUtils.renderIntoDocument(instance);
      expect(spy.calls.count()).toBe(1);
      expect(spy.calls.argsFor(0)[1]).toBe('num');
    });