Remove _BufferList from dart:io and now use BytesBuilder.

sdk/lib/io/websocket_impl.dart

Index: sdk/lib/io/websocket_impl.dart
diff --git a/sdk/lib/io/websocket_impl.dart b/sdk/lib/io/websocket_impl.dart
index d9030619b685c10cd8cf3f954e449fb2d7ab0231..33aec3f51f2fa1e8bd2c56a34e62d6f3b35c6016 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -111,9 +111,9 @@ class _WebSocketProtocolTransformer extends StreamEventTransformer {
               _currentMessageType = _WebSocketMessageType.BINARY;
               _controller = new StreamController(sync: true);
               _controller.stream
-                  .fold(new _BufferList(), (buffer, data) => buffer..add(data))
+                  .fold(new BytesBuilder(), (buffer, data) => buffer..add(data))
                   .then((buffer) {
-                    sink.add(buffer.readBytes());
+                    sink.add(buffer.takeBytes());
                   }, onError: (error) {
                     sink.addError(error);
                   });