Make the HTTP MockClient work with GET requests.

pkg/http/lib/src/utils.dart

Index: pkg/http/lib/src/utils.dart
diff --git a/pkg/http/lib/src/utils.dart b/pkg/http/lib/src/utils.dart
index e75cbd58dcbb8ac333958ea213ae1bd319bbc140..665ee1ef388fdeeedf2c8f45515be8d6e94ae39b 100644
--- a/pkg/http/lib/src/utils.dart
+++ b/pkg/http/lib/src/utils.dart
@@ -133,6 +133,8 @@ Uint8List toUint8List(List<int> input) {
 
 /// Buffers all input from an InputStream and returns it as a future.
 Future<List<int>> consumeInputStream(InputStream stream) {
+  if (stream.closed) return new Future<List<int>>.immediate(<int>[]);
+
   var completer = new Completer<List<int>>();
   /// TODO(nweiz): use BufferList when issue 6409 is fixed
   var buffer = <int>[];