Use BASE64 from package:convert.

lib/src/channel.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
+import 'dart:convert' as convert;
 
 import 'package:async/async.dart';
 import 'package:crypto/crypto.dart';
 import 'package:stream_channel/stream_channel.dart';
 
 import 'copy/web_socket_impl.dart';
 
 /// A [StreamChannel] that communicates over a WebSocket.
 ///
 /// This is implemented by classes that use `dart:io` and `dart:html`. The [new
@@ skipping 41 matching lines @@
   /// Signs a `Sec-WebSocket-Key` header sent by a WebSocket client as part of
   /// the [initial handshake][].
   ///
   /// The return value should be sent back to the client in a
   /// `Sec-WebSocket-Accept` header.
   ///
   /// [initial handshake]: https://tools.ietf.org/html/rfc6455#section-4.2.2
   static String signKey(String key) {
     // We use [codeUnits] here rather than UTF-8-decoding the string because
     // [key] is expected to be base64 encoded, and so will be pure ASCII.
-    return BASE64.encode(sha1.convert((key + webSocketGUID).codeUnits).bytes);
+    return convert.BASE64.encode(
+        sha1.convert((key + webSocketGUID).codeUnits).bytes);
   }
 
   /// Creates a new WebSocket handling messaging across an existing [channel].
   ///
   /// This is a cross-platform constructor; it doesn't use either `dart:io` or
   /// `dart:html`. It's also HTTP-API-agnostic, which means that the initial
   /// [WebSocket handshake][] must have already been completed on the socket
   /// before this is called.
   ///
   /// [protocol] should be the protocol negotiated by this handshake, if any.
@@ skipping 31 matching lines @@
   ///
   /// [closeCode] and [closeReason] are the [close code][] and [reason][] sent
   /// to the remote peer, respectively. If they are omitted, the peer will see
   /// a "no status received" code with no reason.
   ///
   /// [close code]: https://tools.ietf.org/html/rfc6455#section-7.1.5
   /// [reason]: https://tools.ietf.org/html/rfc6455#section-7.1.6
   Future close([int closeCode, String closeReason]) =>
       _webSocket.close(closeCode, closeReason);
 }