code: function DocumentChange(properties) {
                      this.targetIds = [];
                      this.removedTargetIds = [];
                      if (properties)
                          for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                              if (properties[keys[i]] != null)
                                  this[keys[i]] = properties[keys[i]];
                  }
docs: Properties of a DocumentChange.
  @memberof google.firestore.v1
  @interface IDocumentChange
  @property {google.firestore.v1.IDocument|null} [document] DocumentChange document
  @property {Array.<number>|null} [targetIds] DocumentChange targetIds
  @property {Array.<number>|null} [removedTargetIds] DocumentChange removedTargetIds
   
  Constructs a new DocumentChange.
  @memberof google.firestore.v1
  @classdesc Represents a DocumentChange.
  @implements IDocumentChange
  @constructor
  @param {google.firestore.v1.IDocumentChange=} [properties] Properties to set