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.
 *
 * @providesModule ReactPropTypes
 */
'use strict';
var ReactElement = require('ReactElement');
var ReactPropTypeLocationNames = require('ReactPropTypeLocationNames');
var emptyFunction = require('emptyFunction');
var getIteratorFn = require('getIteratorFn');
/**
 * Collection of methods that allow declaration and validation of props that are
 * supplied to React components. Example usage:
 *
 *   var Props = require('ReactPropTypes');
 *   var MyArticle = React.createClass({
 *     propTypes: {
 *       // An optional string prop named "description".
 *       description: Props.string,
 *