Fix analyzer errors and warnings for pkg/http.

pkg/http/lib/src/client.dart

 // Copyright (c) 2012, 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.
 
 library client;
 
 import 'dart:io';
+import 'dart:scalarlist';
 
 import 'base_client.dart';
 import 'base_request.dart';
 import 'io_client.dart';
 import 'streamed_response.dart';
+import 'response.dart';
 import 'utils.dart';
 
 /// The interface for HTTP clients that take care of maintaining persistent
 /// connections across multiple requests to the same server. If you only need to
 /// send a single request, it's usually easier to use [head], [get], [post],
 /// [put], or [delete] instead.
 ///
 /// When creating an HTTP client class with additional functionality, you must
 /// extend [BaseClient] rather than [Client]. In most cases, you can wrap
 /// another instance of [Client] and add functionality on top of that. This
@@ skipping 64 matching lines @@
   Future<Uint8List> readBytes(url, {Map<String, String> headers});
 
   /// Sends an HTTP request and asynchronously returns the response.
   Future<StreamedResponse> send(BaseRequest request);
 
   /// Closes the client and cleans up any resources associated with it. It's
   /// important to close each client when it's done being used; failing to do so
   /// can cause the Dart process to hang.
   void close();
 }