Fix more strong-mode warnings.

lib/http.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A composable, [Future]-based library for making HTTP requests.
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'src/client.dart';
@@ skipping 143 matching lines @@
 ///
 /// This automatically initializes a new [Client] and closes that client once
 /// the request is complete. If you're planning on making multiple requests to
 /// the same server, you should use a single [Client] for all of those requests.
 ///
 /// For more fine-grained control over the request and response, use [Request]
 /// instead.
 Future<Uint8List> readBytes(url, {Map<String, String> headers}) =>
   _withClient((client) => client.readBytes(url, headers: headers));
 
-Future _withClient(Future fn(Client)) {
+Future/*<T>*/ _withClient/*<T>*/(Future/*<T>*/ fn(Client client)) async {
   var client = new Client();
-  var future = fn(client);
-  return future.whenComplete(client.close);
+  try {
+    return await fn(client);
+  } finally {
+    client.close();
+  }
 }