void startSync(String name, {Map arguments})

Start a synchronous operation labeled name. Optionally takes a Map of arguments. This operation must be finished before returning to the event queue.

Source 

static void startSync(String name, {Map arguments}) {
  if (name is! String) {
    throw new ArgumentError.value(name,
                                  'name',
                                  'Must be a String');
  }
  var block = new _SyncBlock._(name, _getTraceClock());
  if (arguments is Map) {
    block.arguments.addAll(arguments);
  }
  _stack.add(block);
}