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.

var propValue = props[propName];
    var propType = getPropType(propValue);
    if (propType !== 'object') {
      var locationName = ReactPropTypeLocationNames[location];
      return new Error(
        `Invalid ${locationName} \`${propFullName}\` of type \`${propType}\` ` +
        `supplied to \`${componentName}\`, expected \`object\`.`
      );
    }
    for (var key in shapeTypes) {
      var checker = shapeTypes[key];
      if (!checker) {
        continue;
      }
      var error = checker(
        propValue,
        key,
        componentName,
        location,
        `${propFullName}.${key}`
      );
      if (error) {
        return error;
      }
    }
    return null;
  }
  return createChainableTypeChecker(validate);
}
function isNode(propValue) {