runWithHttpOverrides< R> method
    Runs body in a fresh Zone using the overrides found in overrides.
Note that overrides should be an instance of a class that extends
HttpOverrides.
Implementation
static R runWithHttpOverrides<R>(R body(), HttpOverrides overrides,
    {ZoneSpecification zoneSpecification, Function onError}) {
  return _asyncRunZoned<R>(body,
      zoneValues: {_httpOverridesToken: overrides},
      zoneSpecification: zoneSpecification,
      onError: onError);
}