Dart object API (#6682)

* dart test scripts - generate with `--gen-object-api`

* Dart object API, pack and unpack methods (WIP)

* Dart flatc - extract Builder code to separate functions to reuse it in Pack()

* Dart flatc - use builder field-building implementation in pack()

* Dart flatc - add pack() as an intance method of the "T" class

* Dart object API - make inner fields unpacked as well

* Dart object API - use pack() when collecting field offsets

* Dart object API - use packOnce() for fields that are structs or vectors of structs

* Dart object API - remove obsolete union support TODO

* Dart object API - minor review changes, test and fixes

* Dart object API - revert packOnce() - not supported by other object API implementations

* Dart object API - update docs

* update dart generated code in tests/ to fix CI failure on ./scripts/check-generated-code.sh

* Dart flatc - fix compilation for old MSVC and c++0x

dart/test/flat_buffers_test.dart

@@ -12,10 +12,12 @@ import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import './monster_test_my_game.example_generated.dart' as example;
+import './monster_test_my_game.example2_generated.dart' as example2;
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(BuilderTest);
+    defineReflectiveTests(ObjectAPITest);
     defineReflectiveTests(CheckOtherLangaugesData);
     defineReflectiveTests(GeneratorTest);
   });
@@ -667,6 +669,70 @@ class BuilderTest {
   }
 }
 
+@reflectiveTest
+class ObjectAPITest {
+  void test_tableStat() {
+    final object1 = example.StatT(count: 3, id: "foo", val: 4);
+    final fbb = Builder();
+    final data = fbb.finish(object1.pack(fbb));
+    final object2 = example.Stat(data).unpack();
+    expect(object2.count, object1.count);
+    expect(object2.id, object1.id);
+    expect(object2.val, object1.val);
+    expect(object2.toString(), object1.toString());
+  }
+
+  void test_tableMonster() {
+    final monster = example.MonsterT()
+      ..pos = example.Vec3T(
+          x: 1,
+          y: 2,
+          z: 3,
+          test1: 4.0,
+          test2: example.Color.Red,
+          test3: example.TestT(a: 1, b: 2))
+      ..mana = 2
+      ..name = 'Monstrous'
+      ..inventory = [24, 42]
+      ..color = example.Color.Green
+      // TODO be smarter for unions and automatically set the `type` field?
+      ..testType = example.AnyTypeId.MyGame_Example2_Monster
+      ..test = example2.MonsterT()
+      ..test4 = [example.TestT(a: 3, b: 4), example.TestT(a: 5, b: 6)]
+      ..testarrayofstring = ["foo", "bar"]
+      ..testarrayoftables = [example.MonsterT(name: 'Oof')]
+      ..enemy = example.MonsterT(name: 'Enemy')
+      ..testarrayofbools = [false, true, false]
+      ..testf = 42.24
+      ..testarrayofsortedstruct = [
+        example.AbilityT(id: 1, distance: 5),
+        example.AbilityT(id: 3, distance: 7)
+      ]
+      ..vectorOfLongs = [5, 6, 7]
+      ..vectorOfDoubles = [8.9, 9.0, 10.1, 11.2]
+      ..anyAmbiguousType = example.AnyAmbiguousAliasesTypeId.M2
+      ..anyAmbiguous = null
+      ..vectorOfEnums = [example.Color.Blue, example.Color.Green]
+      ..signedEnum = example.Race.None;
+
+    final fbBuilder = Builder();
+    final offset = monster.pack(fbBuilder);
+    expect(offset, isNonZero);
+    final data = fbBuilder.finish(offset);
+
+    // TODO currently broken because of struct builder issue, see #6688
+    // final monster2 = example.Monster(data); // Monster (reader)
+    // expect(
+    //     // map Monster => MonsterT, Vec3 => Vec3T, ...
+    //     monster2.toString().replaceAllMapped(
+    //         RegExp('([a-zA-z0-9]+){'), (match) => match.group(1) + 'T{'),
+    //     monster.toString());
+    //
+    // final monster3 = monster2.unpack(); // MonsterT
+    // expect(monster3.toString(), monster.toString());
+  }
+}
+
 class StringListWrapperImpl {
   final BufferContext bp;
   final int offset;