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.

function validate(props, propName, componentName, location, propFullName) {
    if (typeof typeChecker !== 'function') {
      return new Error(
        `Property \`${propFullName}\` of component \`${componentName}\` has invalid PropType notation inside objectOf.`
      );
    }
    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 an object.`
      );
    }
    for (var key in propValue) {
      if (propValue.hasOwnProperty(key)) {
        var error = typeChecker(
          propValue,
          key,
          componentName,
          location,
          `${propFullName}.${key}`
        );
        if (error instanceof Error) {
          return error;
        }
      }
    }