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:

class MyHttpClient implements HttpClient {
  ...
  // An implementation of the HttpClient interface
  ...
}

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