Rip out the last dart:io dependency from 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:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'base_client.dart';
 import 'base_request.dart';
+import 'io.dart' as io;
 import 'io_client.dart';
+import 'response.dart';
 import 'streamed_response.dart';
-import 'response.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
 /// allows all classes implementing [Client] to be mutually composable.
 abstract class Client {
-  /// Creates a new Client using the default implementation. This implementation
-  /// uses an underlying `dart:io` [HttpClient] to make requests.
-  factory Client() => new IOClient();
+  /// Creates a new client.
+  ///
+  /// Currently this will create an [IOClient] if `dart:io` is available and
+  /// throw an [UnsupportedError] otherwise. In the future, it will create a
+  /// [BrowserClient] if `dart:html` is available.
+  factory Client() {
+    io.assertSupported("IOClient");
+    return new IOClient();
+  }
 
   /// 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});
 
   /// Sends an HTTP GET request with the given headers to the given URL, which
   /// can be a [Uri] or a [String].
   ///
@@ skipping 71 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();
 }