HttpOverrides class Null safety

This class facilitates overriding HttpClient with a mock implementation. It should be extended by another class in client code with overrides that construct a mock implementation. The implementation in this base class defaults to the actual HttpClient implementation. For example:

// An implementation of the HttpClient interface
class MyHttpClient implements HttpClient {
  MyHttpClient(SecurityContext? c);

  @override
  noSuchMethod(Invocation invocation) {
    // your implementation here
  }
}

void main() {
  HttpOverrides.runZoned(() {
    // Operations will use MyHttpClient instead of the real HttpClient
    // implementation whenever HttpClient is used.
  }, createHttpClient: (SecurityContext? c) => MyHttpClient(c));
}