code: function TokenRequest(callContext, authenticationContext, clientId, resource, redirectUri) {
    this._log = new Logger('TokenRequest', callContext._logContext);
    this._callContext = callContext;
    this._authenticationContext = authenticationContext;
    this._resource = resource;
    this._clientId = clientId;
    this._redirectUri = redirectUri;
  
    // This should be set at the beginning of getToken
    // functions that have a userId.
    this._userId = null;
  
    this._userRealm = null;
    this._pollingClient = {};
  }
docs: Constructs a new TokenRequest object.
  @constructor
  @private
  @param {object} callContext Contains any context information that applies to the request.
  @param {AuthenticationContext} authenticationContext
  @param {string} resource
  @param {string} clientId
  @param {string} redirectUri