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

tests/standalone/io/web_socket_protocol_processor_test.dart

Index: tests/standalone/io/web_socket_protocol_processor_test.dart
diff --git a/tests/standalone/io/web_socket_protocol_processor_test.dart b/tests/standalone/io/web_socket_protocol_processor_test.dart
index 1a70fa92d03f291004216b9b3ff5d2b64c98ef24..4354ab0d27d03f49fcb25d34d0da2f3096aa7123 100644
--- a/tests/standalone/io/web_socket_protocol_processor_test.dart
+++ b/tests/standalone/io/web_socket_protocol_processor_test.dart
@@ -41,7 +41,7 @@ class WebSocketMessageCollector {
 
   void onMessageData(buffer) {
     if (buffer is String) {
-      buffer = _encodeString(buffer);
+      buffer = UTF8.encode(buffer);
     }
     Expect.listEquals(expectedMessage, buffer);
     messageCount++;