The RequestSink receiving requests on the main isolate.
Applications run during testing are run on the main isolate. When running in this way,
an application will only have one RequestSinkType
receiving HTTP requests. This property is that instance.
If an application is running across multiple isolates, this property is null. See start for more details.
Source
RequestSinkType get mainIsolateSink => server?.sink as RequestSinkType;