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.

var props = {testProp: value};
  var error = declaration(
    props,
    'testProp',
    'testComponent',
    ReactPropTypeLocations.prop
  );
  expect(error).toBe(null);
}
describe('ReactPropTypes', function() {
  beforeEach(function() {
    PropTypes = require('ReactPropTypes');
    React = require('React');
    ReactFragment = require('ReactFragment');
    ReactPropTypeLocations = require('ReactPropTypeLocations');
    ReactTestUtils = require('ReactTestUtils');
  });
  describe('Primitive Types', function() {
    it('should warn for invalid strings', function() {
      typeCheckFail(
        PropTypes.string,
        [],
        'Invalid prop `testProp` of type `array` supplied to ' +
        '`testComponent`, expected `string`.'
      );
      typeCheckFail(
        PropTypes.string,
        false,
        'Invalid prop `testProp` of type `boolean` supplied to ' +
        '`testComponent`, expected `string`.'