Avoid passing a nullable value to Completer<nn-type>.complete (#1015)

This is cleanup work required to start enforcing this with static analysis, as per dart-lang/sdk#53253.

Real quick this issue is that this code is unsafe:

```dart
void f(Completer<int> c, int? i) {
  Future<int>.value(i); // Ouch!
  c.complete(i);        // Ouch!
}
```

pkgs/cupertino_http/lib/src/cupertino_api.dart

@@ -885,7 +885,7 @@ class URLSessionWebSocketTask extends URLSessionTask {
       if (error != null) {
         completer.completeError(error);
       } else {
-        completer.complete(message);
+        completer.complete(message!);
       }
       completionPort.close();
     });