code: @VisibleForTesting
    static Map<String, String> getLocalAppEngineEnvironmentVariables(String gaeRuntime) {
      Map<String, String> environment = Maps.newHashMap();
  
      String gaeEnv = "localdev";
      environment.put("GAE_ENV", gaeEnv);
      environment.put("GAE_RUNTIME", gaeRuntime);
  
      return environment;
    }
docs: Gets a {@code Map<String, String>} of the environment variables for running the {@link
  DevServer}.
  
  @param gaeRuntime the runtime ID to set the environment variable GAE_RUNTIME to
  @return {@code Map<String, String>} that maps from the environment variable name to its value