Support a user-supplied dart:io client in pkg/http.

pkg/http/lib/src/io.dart

 // Copyright (c) 2014, 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 http.io;
 
 @MirrorsUsed(targets: const ['dart.io.HttpClient', 'dart.io.HttpException',
   'dart.io.File'])
 import 'dart:mirrors';
 
@@ skipping 25 matching lines @@
 
 /// Creates a new `dart:io` HttpClient instance.
 newHttpClient() => _httpClient.newInstance(const Symbol(''), []).reflectee;
 
 /// Creates a new `dart:io` File instance with the given [path].
 newFile(String path) => _file.newInstance(const Symbol(''), [path]).reflectee;
 
 /// Returns whether [error] is a `dart:io` HttpException.
 bool isHttpException(error) => reflect(error).type.isSubtypeOf(_httpException);
 
+/// Returns whether [client] is a `dart:io` HttpClient.
+bool isHttpClient(client) => reflect(client).type.isSubtypeOf(_httpClient);
+
 /// Tries to load `dart:io` and returns `null` if it fails.
 LibraryMirror _getLibrary() {
   try {
     return currentMirrorSystem().findLibrary(const Symbol('dart.io'));
   } catch (_) {
     // TODO(nweiz): narrow the catch clause when issue 18532 is fixed.
     return null;
   }
 }