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

pkg/http/lib/src/io_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 io_client;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'base_client.dart';
 import 'base_request.dart';
 import 'exception.dart';
 import 'io.dart' as io;
 import 'streamed_response.dart';
 
 /// A `dart:io`-based HTTP client. This is the default client.
 class IOClient extends BaseClient {
   /// The underlying `dart:io` HTTP client.
   var _inner;
 
   /// Creates a new HTTP client.
-  IOClient() {
+  ///
+  /// [innerClient] must be a `dart:io` HTTP client. If it's not passed, a
+  /// default one will be instantiated.
+  IOClient([innerClient]) {
     io.assertSupported("IOClient");
-    _inner = io.newHttpClient();
+    if (innerClient != null) {
+      // TODO(nweiz): remove this assert when we can type [innerClient]
+      // properly.
+      assert(io.isHttpClient(innerClient));
+      _inner = innerClient;
+    } else {
+      _inner = io.newHttpClient();
+    }
   }
 
   /// Sends an HTTP request and asynchronously returns the response.
   Future<StreamedResponse> send(BaseRequest request) {
     var stream = request.finalize();
 
     return Chain.track(_inner.openUrl(request.method, request.url))
         .then((ioRequest) {
       var contentLength = request.contentLength == null ?
           -1 : request.contentLength;
@@ skipping 31 matching lines @@
     });
   }
 
   /// Closes the client. This terminates all active connections. If a client
   /// remains unclosed, the Dart process may not terminate.
   void close() {
     if (_inner != null) _inner.close(force: true);
     _inner = null;
   }
 }