See comment here: https://github.com/facebook/react/pull/11457#discussion_r148954193.

Although it seems very unlikely that someone would pass a one-character prop, the price (at least in versions of V8 in Chrome right now) is reported to be about 10x.

So we should check `.length` first and then do the comparison.

**Please comment if you want to take this issue.**
**If somebody already commented, please let them finish it.**
* DOMAttributeNames: object mapping React attribute name to the DOM
   * attribute name. Attribute names not specified use the **lowercase**
   * normalized name.
   *
   * DOMAttributeNamespaces: object mapping React attribute name to the DOM
   * attribute namespace URL. (Attribute names not specified use no namespace.)
   *
   * DOMPropertyNames: similar to DOMAttributeNames but for DOM properties.
   * Property names not specified use the normalized name.
   *
   * DOMMutationMethods: Properties that require special mutation methods. If
   * `value` is undefined, the mutation method should unset the property.
   *
   * @param {object} domPropertyConfig the config as described above.
   */
  injectDOMPropertyConfig: function(domPropertyConfig) {
    var Injection = DOMPropertyInjection;
    var Properties = domPropertyConfig.Properties || {};
    var DOMAttributeNamespaces = domPropertyConfig.DOMAttributeNamespaces || {};
    var DOMAttributeNames = domPropertyConfig.DOMAttributeNames || {};
    var DOMMutationMethods = domPropertyConfig.DOMMutationMethods || {};
    for (var propName in Properties) {
      invariant(
        !properties.hasOwnProperty(propName),
        "injectDOMPropertyConfig(...): You're trying to inject DOM property " +
          "'%s' which has already been injected. You may be accidentally " +
          'injecting the same DOM property config twice, or you may be ' +
          'injecting two configs that have conflicting property names.',
        propName,
      );