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.

*       // A required enum prop named "category".
 *       category: Props.oneOf(['News','Photos']).isRequired,
 *
 *       // A prop named "dialog" that requires an instance of Dialog.
 *       dialog: Props.instanceOf(Dialog).isRequired
 *     },
 *     render: function() { ... }
 *   });
 *
 * A more formal specification of how these methods are used:
 *
 *   type := array|bool|func|object|number|string|oneOf([...])|instanceOf(...)
 *   decl := ReactPropTypes.{type}(.isRequired)?
 *
 * Each and every declaration produces a function with the same signature. This
 * allows the creation of custom validation functions. For example:
 *
 *  var MyLink = React.createClass({
 *    propTypes: {
 *      // An optional string or URI prop named "href".
 *      href: function(props, propName, componentName) {
 *        var propValue = props[propName];
 *        if (propValue != null && typeof propValue !== 'string' &&
 *            !(propValue instanceof URI)) {
 *          return new Error(
 *            'Expected a string or an URI for ' + propName + ' in ' +
 *            componentName
 *          );
 *        }
 *      }