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.

return null;
  }
  return createChainableTypeChecker(validate);
}
function createUnionTypeChecker(arrayOfTypeCheckers) {
  if (!Array.isArray(arrayOfTypeCheckers)) {
    return createChainableTypeChecker(function() {
      return new Error(
        `Invalid argument supplied to oneOfType, expected an instance of array.`
      );
    });
  }
  function validate(props, propName, componentName, location, propFullName) {
    for (var i = 0; i < arrayOfTypeCheckers.length; i++) {
      var checker = arrayOfTypeCheckers[i];
      if (
        checker(props, propName, componentName, location, propFullName) == null
      ) {
        return null;
      }
    }
    var locationName = ReactPropTypeLocationNames[location];
    return new Error(
      `Invalid ${locationName} \`${propFullName}\` supplied to ` +
      `\`${componentName}\`.`
    );
  }
  return createChainableTypeChecker(validate);
}
function createNodeChecker() {