code: function prototypeMethodOrNull(className, method) {
    var ctor = goog.global[className];
    return (ctor && ctor.prototype && ctor.prototype[method]) || null;
  }
docs: Shorthand for `DOMInterface.prototype.method` to improve readability
  during initialization of `Methods`.
  @param {string} className
  @param {string} method
  @return {?Function}