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.

switch (typeof propValue) {
    case 'number':
    case 'string':
    case 'undefined':
      return true;
    case 'boolean':
      return !propValue;
    case 'object':
      if (Array.isArray(propValue)) {
        return propValue.every(isNode);
      }
      if (propValue === null || ReactElement.isValidElement(propValue)) {
        return true;
      }
      var iteratorFn = getIteratorFn(propValue);
      if (iteratorFn) {
        var iterator = iteratorFn.call(propValue);
        var step;
        if (iteratorFn !== propValue.entries) {
          while (!(step = iterator.next()).done) {
            if (!isNode(step.value)) {
              return false;
            }
          }
        } else {
          
          while (!(step = iterator.next()).done) {
            var entry = step.value;
            if (entry) {