Remove Encoding-enum from dart:io and add interface in dart:convert.

tests/standalone/io/web_socket_protocol_processor_test.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.
 
 import "package:expect/expect.dart";
 import "dart:convert";
 import "dart:math";
 import "dart:async";
 import "dart:collection";
 import "dart:typed_data";
@@ skipping 23 matching lines @@
 
   Function onClosed;
 
   WebSocketMessageCollector(Stream stream,
                             [List<int> this.expectedMessage = null]) {
     stream.listen(onMessageData, onDone: onClosed, onError: onError);
   }
 
   void onMessageData(buffer) {
     if (buffer is String) {
-      buffer = _encodeString(buffer);
+      buffer = UTF8.encode(buffer);
     }
     Expect.listEquals(expectedMessage, buffer);
     messageCount++;
     data = buffer;
   }
 
   void onError(e) {
     String msg = "Unexpected error $e";
     var trace = getAttachedStackTrace(e);
     if (trace != null) msg += "\nStackTrace: $trace";
@@ skipping 186 matching lines @@
       true, FRAME_OPCODE_BINARY, null, message, 0, message.length);
   controller.add(frame);
 }
 
 
 void main() {
   testFullMessages();
   testFragmentedMessages();
   testUnmaskedMessage();
 }