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 (_isProduct) {
    return;
  }
  if (name is! String) {
    throw new ArgumentError.value(name,
                                  'name',
                                  'Must be a String');
  }
  if (!_isDartStreamEnabled()) {
    // Push a null onto the stack and return.
    _stack.add(null);
    return;
  }
  var block = new _SyncBlock._(name, _getTraceClock(), _getThreadCpuClock());
  if (arguments is Map) {
    block._appendArguments(arguments);
  }
  _stack.add(block);
}