message: Dart - fixup writeString() return type after previous merges
diff: @@ -668,7 +668,7 @@ class Builder {
  return result;
  }
  - /// Write the given string [value] and return its offset
  + /// Write the given string [value] and return its offset.
  ///
  /// Dart strings are UTF-16 but must be stored as UTF-8 in FlatBuffers.
  /// If the given string consists only of ASCII characters, you can indicate
  @@ -676,7 +676,7 @@ class Builder {
  /// copy the ASCII string directly to the output buffer and if that fails
  /// (because there are no-ASCII characters in the string) it falls back and to
  /// the default UTF-16 -> UTF-8 conversion (with slight performance penalty).
  - int? writeString(String value, {bool asciiOptimization = false}) {
  + int writeString(String value, {bool asciiOptimization = false}) {
  _ensureNoVTable();
  if (_strings != null) {
  return _strings!
  