d.elements);
    }, superClass);
    var decorated = api.decorateClass(
      _coalesceClassElements(r.d.map(_createElementDescriptor)),
      decorators,
    );

    api.initializeClassElements(r.F, decorated.elements);

    return api.runClassFinishers(r.F, decorated.finishers);
  }

  function _getDecoratorsApi() {
    _getDecoratorsApi = function() {
      return api;
    };

    var api = {
      elementsDefinitionOrder: [["method"], ["field"]],

      // InitializeInstanceElements
      initializeInstanceElements: function(
        /*::<C>*/ O /*: C */,
        elements /*: ElementDescriptor[] */,
      ) {
        ["method", "field"].forEach(function(kind) {
          elements.forEach(function(element /*: ElementDescriptor */) {
            if (element.kind === kind && element.placement === "own") {
              this.defineClassElement(O, element);
            }
          }, this);
        }, this);
      },

      // InitializeClassElements
      initializeClassElements: function(
        /*::<C>*/ F /*: Class<C> */,
        elements /*: ElementDescriptor[] */,
      ) {
        var proto = F.prototype;

        ["method", "field"].forEach(function(kind) {
          elements.forEach(function(element /*: ElementDescriptor */) {
            var placement = element.placement;
            if (
              element.kind === kind &&
              (placement === "static" || placement === "prototype")
            ) {
              var receiver = placement === "static" ? F : proto;
              this.defineClassElement(receiver, element);
            }
          }, this);
        }, this);
      },

      // DefineClassElement
      defineClassElement: function(
        /*::<C>*/ receiver /*: C | Class<C> */,
        element /*: ElementDescriptor */,
      ) {
        var descriptor /*: PropertyDescriptor */ = element.descriptor;
        if (element.kind === "field") {
          var initializer = element.initializer;
          descriptor = {
            enumerable: descriptor.enumerable,
            writable: descriptor.writable,
            configurable: descriptor.configurable,
            value: initializer === void 0 ? void 0 : initializer.call(receiver),
          };
        }
        Object.defineProperty(receiver, element.key, descriptor);
      },

      // DecorateClass
      decorateClass: function(
        elements /*: ElementDescriptor[] */,
        decorators /*: ClassDecorator[] */,
      ) /*: ElementsFinishers */ {
        var newElements /*: ElementDescriptor[] */ = [];
        var finishers /*: ClassFinisher[] */ = [];
        var placements /*: Placements */ = {
          static: [],
          prototype: [],
          own: [],
        };

        elements.forEach(function(element /*: ElementDescriptor */) {
          this.addElementPlacement(element, placements);
        }, this);

        elements.forEach(function(element /*: ElementDescriptor */) {
          if (!_hasDecorators(element)) return newElements.push(element);

          var elementFinishersExtras /*: ElementFinishersExtras */ = this.decorateElement(
            element,
            placements,
          );
          newElements.push(elementFinishersExtras.element);
          newElements.push.apply(newElements, elementFinishersExtras.extras);
          finishers.push.apply(finishers, elementFinishersExtras.finishers);
        }, this);

        if (!decorators) {
          return { elements: newElements, finishers: finishers };
        }

        var result /*: ElementsFinishers */ = this.decorateConstructor(
          newElements,
          decorators,
        );
        finishers.push.apply(finishers, result.finishers);
        result.finishers = finishers;

        return result;
      },

      // AddElementPlacement
      addElementPlacement: function(
        element /*: ElementDescriptor */,
        placements /*: Placements */,
        silent /*: boolean */,
      ) {
        var keys = placements[element.placement];
        if (!silent && keys.indexOf(element.key) !== -1) {
          throw new TypeError("Duplicated element (" + element.key + ")");
        }
        keys.push(element.key);
      },

      // DecorateElement
      decorateElement: function(
        element /*: ElementDescriptor */,
        placements /*: Placements */,
      ) /*: ElementFinishersExtras */ {
        var extras /*: ElementDescriptor[] */ = [];
        var finishers /*: ClassFinisher[] */ = [];

        for (
          var decorators = element.decorators, i = decorators.length - 1;
          i >= 0;
          i--
        ) {
          // (inlined) RemoveElementPlacement
          var keys = placements[element.placement];
          keys.splice(keys.indexOf(element.key), 1);

          var elementObject /*: ElementObjectInput */ = this.fromElementDescriptor(
            element,
          );
          var elementFinisherExtras /*: ElementFinisherExtras */ = this.toElementFinisherExtras(
            (0, decorators[i])(elementObject) /*: ElementObjectOutput */ ||
              elementObject,
          );

          element = elementFinisherExtras.element;
          this.addElementPlacement(element, placements);

          if (elementFinisherExtras.finisher) {
            finishers.push(elementFinisherExtras.finisher);
          }

          var newExtras /*: ElementDescriptor[] | void */ =
            elementFinisherExtras.extras;
          if (newExtras) {
            for (var j = 0; j < newExtras.length; j++) {
              this.addElementPlacement(newExtras[j], placements);
            }
            extras.push.apply(extras, newExtras);
          }
        }

        return { element: element, finishers: finishers, extras: extras };
      },

      // DecorateConstructor
      decorateConstructor: function(
        elements /*: ElementDescriptor[] */,
        decorators /*: ClassDecorator[] */,
      ) /*: ElementsFinishers */ {
        var finishers /*: ClassFinisher[] */ = [];

        for (var i = decorators.length - 1; i >= 0; i--) {
          var obj /*: ClassObject */ = this.fromClassDescriptor(elements);
          var elementsAndFinisher /*: ElementsFinisher */ = this.toClassDescriptor(
            (0, decorators[i])(obj) /*: ClassObject */ || obj,
          );

          if (elementsAndFinisher.finisher !== undefined) {
            finishers.push(elementsAndFinisher.finisher);
          }

          if (elementsAndFinisher.elements !== undefined) {
            elements = elementsAndFinisher.elements;

            for (var j = 0; j < elements.length - 1; j++) {
              for (var k = j + 1; k < elements.length; k++) {
                if (
                  elements[j].key === elements[k].key &&
                  elements[j].placement === elements[k].placement
                ) {
                  throw new TypeError(
                    "Duplicated element (" + elements[j].key + ")",
                  );
                }
              }
            }
          }
        }

        return { elements: elements, finishers: finishers };
      },

      // FromElementDescriptor
      fromElementDescriptor: function(
        element /*: ElementDescriptor */,
      ) /*: ElementObject */ {
        var obj /*: ElementObject */ = {
          kind: element.kind,
          key: element.key,
          placement: element.placement,
          descriptor: element.descriptor,
        };

        var desc = {
          value: "Descriptor",
          configurable: true,
        };
        Object.defineProperty(obj, Symbol.toStringTag, desc);

        if (element.kind === "field") obj.initializer = element.initializer;

        return obj;
      },

      // ToElementDescriptors
      toElementDescriptors: function(
        elementObjects /*: ElementObject[] */,
      ) /*: ElementDescriptor[] */ {
        if (elementObjects === undefined) return;
        return toArray(elementObjects).map(function(elementObject) {
          var element = this.toElementDescriptor(elementObject);
          this.disallowProperty(elementObject, "finisher", "An element descriptor");
          this.disallowProperty(elementObject, "extras", "An element descriptor");
          return element;
        }, this);
      },

      // ToElementDescriptor
      toElementDescriptor: function(
        elementObject /*: ElementObject */,
      ) /*: ElementDescriptor */ {
        var kind = String(elementObject.kind);
        if (kind !== "method" && kind !== "field") {
          throw new TypeError(
            'An element descriptor\\'s .kind property must be either "method" or' +
              ' "field", but a decorator created an element descriptor with' +
              ' .kind "' +
              kind +
              '"',
          );
        }

        var key = toPropertyKey(elementObject.key);

        var placement = String(elementObject.placement);
        if (
          placement !== "static" &&
          placement !== "prototype" &&
          placement !== "own"
        ) {
          throw new TypeError(
            'An element descriptor\\'s .placement property must be one of "static",' +
              ' "prototype" or "own", but a decorator created an element descriptor' +
              ' with .placement "' +
              placement +
              '"',
          );
        }

        var descriptor /*: PropertyDescriptor */ = elementObject.descriptor;

        this.disallowProperty(elementObject, "elements", "An element descriptor");

        var element /*: ElementDescriptor */ = {
          kind: kind,
          key: key,
          placement: placement,
          descriptor: Object.assign({}, descriptor),
        };

        if (kind !== "field") {
          this.disallowProperty(elementObject, "initializer", "A method descriptor");
        } else {
          this.disallowProperty(
            descriptor,
            "get",
            "The property descriptor of a field descriptor",
          );
          this.disallowProperty(
            descriptor,
            "set",
            "The property descriptor of a field descriptor",
          );
          this.disallowProperty(
            descriptor,
            "value",
            "The property descriptor of a field descriptor",
          );

          element.initializer = elementObject.initializer;
        }

        return element;
      },

      toElementFinisherExtras: function(
        elementObject /*: ElementObject */,
      ) /*: ElementFinisherExtras */ {
        var element /*: ElementDescriptor */ = this.toElementDescriptor(
          elementObject,
        );
        var finisher /*: ClassFinisher */ = _optionalCallableProperty(
          elementObject,
          "finisher",
        );
        var extras /*: ElementDescriptors[] */ = this.toElementDescriptors(
          elementObject.extras,
        );

        return { element: element, finisher: finisher, extras: extras };
      },

      // FromClassDescriptor
      fromClassDescriptor: function(
        elements /*: ElementDescriptor[] */,
      ) /*: ClassObject */ {
        var obj = {
          kind: "class",
          elements: elements.map(this.fromElementDescriptor, this),
        };

        var desc = { value: "Descriptor", configurable: true };
        Object.defineProperty(obj, Symbol.toStringTag, desc);

        return obj;
      },

      // ToClassDescriptor
      toClassDescriptor: function(
        obj /*: ClassObject */,
      ) /*: ElementsFinisher */ {
        var kind = String(obj.kind);
        if (kind !== "class") {
          throw new TypeError(
            'A class descriptor\\'s .kind property must be "class", but a decorator' +
              ' created a class descriptor with .kind "' +
              kind +
              '"',
          );
        }

        this.disallowProperty(obj, "key", "A class descriptor");
        this.disallowProperty(obj, "placement", "A class descriptor");
        this.disallowProperty(obj, "descriptor", "A class descriptor");
        this.disallowProperty(obj, "initializer", "A class descriptor");
        this.disallowProperty(obj, "extras", "A class descriptor");

        var finisher = _optionalCallableProperty(obj, "finisher");
        var elements = this.toElementDescriptors(obj.elements);

        return { elements: elements, finisher: finisher };
      },

      // RunClassFinishers
      runClassFinishers: function(
        constructor /*: Class<*> */,
        finishers /*: ClassFinisher[] */,
      ) /*: Class<*> */ {
        for (var i = 0; i < finishers.length; i++) {
          var newConstructor /*: ?Class<*> */ = (0, finishers[i])(constructor);
          if (newConstructor !== undefined) {
            // NOTE: This should check if IsConstructor(newConstructor) is false.
            if (typeof newConstructor !== "function") {
              throw new TypeError("Finishers must return a constructor.");
            }
            constructor = newConstructor;
          }
        }
        return constructor;
      },

      disallowProperty: function(obj, name, objectType) {
        if (obj[name] !== undefined) {
          throw new TypeError(objectType + " can't have a ." + name + " property.");
        }
      }
    };

    return api;
  }

  // ClassElementEvaluation
  function _createElementDescriptor(
    def /*: ElementDefinition */,
  ) /*: ElementDescriptor */ {
    var key = toPropertyKey(def.key);

    var descriptor /*: PropertyDescriptor */;
    if (def.kind === "method") {
      descriptor = {
        value: def.value,
        writable: true,
        configurable: true,
        enumerable: false,
      };
    } else if (def.kind === "get") {
      descriptor = { get: def.value, configurable: true, enumerable: false };
    } else if (def.kind === "set") {
      descriptor = { set: def.value, configurable: true, enumerable: false };
    } else if (def.kind === "field") {
      descriptor = { configurable: true, writable: true, enumerable: true };
    }

    var element /*: ElementDescriptor */ = {
      kind: def.kind === "field" ? "field" : "method",
      key: key,
      placement: def.static
        ? "static"
        : def.kind === "field"
        ? "own"
        : "prototype",
      descriptor: descriptor,
    };
    if (def.decorators) element.decorators = def.decorators;
    if (def.kind === "field") element.initializer = def.value;

    return element;
  }

  // CoalesceGetterSetter
  function _coalesceGetterSetter(
    element /*: ElementDescriptor */,
    other /*: ElementDescriptor */,
  ) {
    if (element.descriptor.get !== undefined) {
      other.descriptor.get = element.descriptor.get;
    } else {
      other.descriptor.set = element.descriptor.set;
    }
  }

  // CoalesceClassElements
  function _coalesceClassElements(
    elements /*: ElementDescriptor[] */,
  ) /*: ElementDescriptor[] */ {
    var newElements /*: ElementDescriptor[] */ = [];

    var isSameElement = function(
      other /*: ElementDescriptor */,
    ) /*: boolean */ {
      return (
        other.kind === "method" &&
        other.key === element.key &&
        other.placement === element.placement
      );
    };

    for (var i = 0; i < elements.length; i++) {
      var element /*: ElementDescriptor */ = elements[i];
      var other /*: ElementDescriptor */;

      if (
        element.kind === "method" &&
        (other = newElements.find(isSameElement))
      ) {
        if (
          _isDataDescriptor(element.descriptor) ||
          _isDataDescriptor(other.descriptor)
        ) {
          if (_hasDecorators(element) || _hasDecorators(other)) {
            throw new ReferenceError(
              "Duplicated methods (" + element.key + ") can't be decorated.",
            );
          }
          other.descriptor = element.descriptor;
        } else {
          if (_hasDecorators(element)) {
            if (_hasDecorators(other)) {
              throw new ReferenceError(
                "Decorators can't be placed on different accessors with for " +
                  "the same property (" +
                  element.key +
                  ").",
              );
            }
            other.decorators = element.decorators;
          }
          _coalesceGetterSetter(element, other);
        }
      } else {
        newElements.push(element);
      }
    }

    return newElements;
  }

  function _hasDecorators(element /*: ElementDescriptor */) /*: boolean */ {
    return element.decorators && element.decorators.length;
  }

  function _isDataDescriptor(desc /*: PropertyDescriptor */) /*: boolean */ {
    return (
      desc !== undefined &&
      !(desc.value === undefined && desc.writable === undefined)
    );
  }

  function _optionalCallableProperty /*::<T>*/(
    obj /*: T */,
    name /*: $Keys<T> */,
  ) /*: ?Function */ {
    var value = obj[name];
    if (value !== undefined && typeof value !== "function") {
      throw new TypeError("Expected '" + name + "' to be a function");
    }
    return value;
  }