code: function Target(properties) {
                      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 Target.
  @memberof google.firestore.v1
  @interface ITarget
  @property {google.firestore.v1.Target.IQueryTarget|null} [query] Target query
  @property {google.firestore.v1.Target.IDocumentsTarget|null} [documents] Target documents
  @property {Uint8Array|null} [resumeToken] Target resumeToken
  @property {google.protobuf.ITimestamp|null} [readTime] Target readTime
  @property {number|null} [targetId] Target targetId
  @property {boolean|null} [once] Target once
   
  Constructs a new Target.
  @memberof google.firestore.v1
  @classdesc Represents a Target.
  @implements ITarget
  @constructor
  @param {google.firestore.v1.ITarget=} [properties] Properties to set