message: Graphics class has a property called "isMask" which is never actually set
diff: @@ -589,6 +589,7 @@ export default class DisplayObject extends EventEmitter
  if (this._mask)
  {
  this._mask.renderable = true;
  + this._mask.isMask = false;
  }
  this._mask = value;
  @@ -596,6 +597,7 @@ export default class DisplayObject extends EventEmitter
  if (this._mask)
  {
  this._mask.renderable = false;
  + this._mask.isMask = true;
  }
  }
  