[dart] fix bug which generated wrong code

[fawdlstty]

[recommit] #8541

Because the [#8380] author hasn't updated the working tree for a long time, I decided to reopen [#8259]

[fawdlstty]

Hello @aardappel . Sorry, I'm bother you again.
I re-submitted this code. After running the command "generate_code.py", the output was:

E:\GitHub_fa\flatbuffers\scripts>python generate_code.py
E:\GitHub_fa\flatbuffers\flatc.exe: 
warning:
  cannot generate code directly from .proto files

Fields in struct ProtoMessage have gap between ids
Fields in struct OtherMessage have gap between ids
When you use --proto, that you should check for conformity yourself, using the existing --conform
warning:
  E:\GitHub_fa\flatbuffers\tests\rust_namer_test.fbs(26, 8): warning: field names should be lowercase snake_case, got: Message


E:\GitHub_fa\flatbuffers\scripts>

It seems that it has nothing to do with my revisions. Is this PR okay?

[aardappel]

index b9f7640..4f5d464 100644
--- a/dart/test/keyword_test_keyword_test_generated.dart
+++ b/dart/test/keyword_test_keyword_test_generated.dart
@@ -7,6 +7,7 @@ import 'dart:typed_data' show Uint8List;
 import 'package:flat_buffers/flat_buffers.dart' as fb;
 
 
+
 enum Abc {
   $void(0),
   where(1),
diff --git a/tests/monster_extra_my_game_generated.dart b/tests/monster_extra_my_game_generated.dart
index 77ce07f..833dfe5 100644
--- a/tests/monster_extra_my_game_generated.dart
+++ b/tests/monster_extra_my_game_generated.dart
@@ -7,6 +7,7 @@ import 'dart:typed_data' show Uint8List;
 import 'package:flat_buffers/flat_buffers.dart' as fb;
 
 
+
 class MonsterExtra {
   MonsterExtra._(this._bc, this._bcOffset);
   factory MonsterExtra(List<int> bytes) {

ERROR: ********************************************************
ERROR: * The following differences were found after running   *
ERROR: * the script/generate_code.py script. Maybe you forgot *
ERROR: * to run it after making changes in a generator?       *
ERROR: ********************************************************```

[jtdavis777]

hey @fawdlstty - happy to get this merged, if you could take care of the generated code diff mentioned above.