Fix Socket_WriteList to use the offset for external byte arrays.

runtime/bin/socket.cc

Index: runtime/bin/socket.cc
diff --git a/runtime/bin/socket.cc b/runtime/bin/socket.cc
index 1285f1363a17353ac579cf555048d2cefd2c5e12..dfde8f3e8434d8ab03af79e5afc1101628d572ee 100644
--- a/runtime/bin/socket.cc
+++ b/runtime/bin/socket.cc
@@ -186,6 +186,7 @@ void FUNCTION_NAME(Socket_WriteList)(Dart_NativeArguments args) {
     if (Dart_IsError(result)) {
       Dart_PropagateError(result);
     }
+    buffer = static_cast<void*>((static_cast<uint8_t*>(buffer) + offset));
     bytes_written = Socket::Write(socket, buffer, length);
     if (bytes_written > 0) total_bytes_written = bytes_written;
   } else {