* Prepare for HttpClientResponse Uint8List SDK change

An upcoming change in the Dart SDK will change `HttpClientResponse`
from implementing `Stream<List<int>>` to instead implement
`Stream<Uint8List>`.

This forwards-compatible change to `_MockHttpClientResponse` is being
made to allow for a smooth rollout of that SDK breaking change. The
current structure of the class is as follows:

```dart
_MockHttpClientResponse extends Stream<List<int>> implements HttpClientResponse {
  ...
}
```

This structure would require that the Dart SDK change land atomically
a change to the class (`extends Stream<Uint8List>`). This atomic landing
requirement doesn't play well at all with Flutter's roll model vis-a-vis
the Dart SDK's roll model to Google's internal repo.  As such, this commit
changes the structure of `_MockHttpClientResponse` to be:

```dart
_MockHttpClientResponse implements HttpClientResponse {
  final Stream<Uint8List> _delegate;

  ...
}
```

Once the Dart SDK change has fully rolled out, we can simplify this class
back to its former structure.

https://github.com/dart-lang/sdk/issues/36900

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