code: public void login(String user) throws AppEngineException {
      Preconditions.checkNotNull(user);
      if (!EMAIL_PATTERN.matcher(user).find()) {
        throw new AppEngineException("Invalid email address: " + user);
      }
      try {
        runner.run(ImmutableList.of("auth", "login", user), null);
      } catch (ProcessHandlerException | IOException ex) {
        throw new AppEngineException(ex);
      }
    }
docs: Logs into the Cloud SDK with a specific user (does not retrigger auth flow if user is already
  configured for the system).
  
  @param user a user email
  @throws AppEngineException when there is an issue with the auth flow