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.

k1: 'Some string',
        k2: <div />,
        k3: frag({
          k30: <MyComponent />,
          k31: frag({k310: <a />}),
          k32: 'Another string',
        }),
        k4: null,
        k5: undefined,
      }));
      expect(console.error.calls.count()).toBe(0);
    });
    it('should not warn for iterables', function() {
      var iterable = {
        '@@iterator': function() {
          var i = 0;
          return {
            next: function() {
              var done = ++i > 2;
              return {value: done ? undefined : <MyComponent />, done: done};
            },
          };
        },
      };
      typeCheckPass(PropTypes.node, iterable);
    });
    it('should not warn for entry iterables', function() {
      var iterable = {
        '@@iterator': function() {
          var i = 0;