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.

/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @emails react-core
 */
'use strict';
var PropTypes;
var React;
var ReactFragment;
var ReactPropTypeLocations;
var ReactTestUtils;
var Component;
var MyComponent;
var requiredMessage =
  'Required prop `testProp` was not specified in `testComponent`.';
function typeCheckFail(declaration, value, message) {