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

tests/monster_test_my_game.example2_generated.dart

@@ -26,6 +26,27 @@ class Monster {
   String toString() {
     return 'Monster{}';
   }
+
+  MonsterT unpack() => MonsterT();
+
+  static int pack(fb.Builder fbBuilder, MonsterT object) {
+    if (object == null) return 0;
+    return object.pack(fbBuilder);
+  }
+}
+
+class MonsterT {
+  int pack(fb.Builder fbBuilder) {
+    assert(fbBuilder != null);
+
+    fbBuilder.startTable();
+    return fbBuilder.endTable();
+  }
+
+  @override
+  String toString() {
+    return 'MonsterT{}';
+  }
 }
 
 class _MonsterReader extends fb.TableReader<Monster> {