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.

if (!Array.isArray(expectedValues)) {
    return createChainableTypeChecker(function() {
      return new Error(
        `Invalid argument supplied to oneOf, expected an instance of array.`
      );
    });
  }
  function validate(props, propName, componentName, location, propFullName) {
    var propValue = props[propName];
    for (var i = 0; i < expectedValues.length; i++) {
      if (is(propValue, expectedValues[i])) {
        return null;
      }
    }
    var locationName = ReactPropTypeLocationNames[location];
    var valuesString = JSON.stringify(expectedValues);
    return new Error(
      `Invalid ${locationName} \`${propFullName}\` of value \`${propValue}\` ` +
      `supplied to \`${componentName}\`, expected one of ${valuesString}.`
    );
  }
  return createChainableTypeChecker(validate);
}
function createObjectOfTypeChecker(typeChecker) {