Rip out the last dart:io dependency from 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 'dart:io';
 
 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.
-  HttpClient _inner;
+  var _inner;
 
   /// Creates a new HTTP client.
-  IOClient() : _inner = new HttpClient();
+  IOClient() {
+    io.assertSupported("IOClient");
+    _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;
       ioRequest
           ..followRedirects = request.followRedirects
           ..maxRedirects = request.maxRedirects
           ..contentLength = contentLength
           ..persistentConnection = request.persistentConnection;
       request.headers.forEach((name, value) {
         ioRequest.headers.set(name, value);
       });
       return Chain.track(stream.pipe(ioRequest));
     }).then((response) {
       var headers = {};
       response.headers.forEach((key, values) {
         headers[key] = values.join(',');
       });
 
       var contentLength = response.contentLength == -1 ?
           null : response.contentLength;
       return new StreamedResponse(
           response.handleError((error) =>
               throw new ClientException(error.message, error.uri),
-              test: (error) => error is HttpException),
+              test: (error) => io.isHttpException(error)),
           response.statusCode,
           contentLength: contentLength,
           request: request,
           headers: headers,
           isRedirect: response.isRedirect,
           persistentConnection: response.persistentConnection,
           reasonPhrase: response.reasonPhrase);
     }).catchError((error) {
-      if (error is! HttpException) throw error;
+      if (!io.isHttpException(error)) throw error;
       throw new ClientException(error.message, error.uri);
     });
   }
 
   /// 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;
   }
 }