Refactor http.Client and http.BaseClient to allow http.Client in type annotations.

pkg/http/lib/src/base_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 base_client;
 
 import 'dart:io';
 import 'dart:scalarlist';
 import 'dart:uri';
 
 import 'base_request.dart';
+import 'client.dart';
 import 'request.dart';
 import 'response.dart';
 import 'streamed_response.dart';
 import 'utils.dart';
 
 /// The abstract base class for an HTTP client. This is a mixin-style class;
 /// subclasses only need to implement [send] and maybe [close], and then they
 /// get various convenience methods for free.
-abstract class BaseClient {
+abstract class BaseClient implements Client {
   /// Sends an HTTP HEAD request with the given headers to the given URL, which
   /// can be a [Uri] or a [String].
   ///
   /// For more fine-grained control over the request, use [send] instead.
   Future<Response> head(url, {Map<String, String> headers}) =>
     _sendUnstreamed("HEAD", url, headers);
 
   /// Sends an HTTP GET request with the given headers to the given URL, which
   /// can be a [Uri] or a [String].
   ///
@@ skipping 95 matching lines @@
       message = "$message: ${response.reasonPhrase}";
     }
     throw new HttpException("$message.");
   }
 
   /// 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() {}
 }