Add [Dart] support (#4676)

* Add [Dart] support

* fix enum vectors

* Allow for opt out of string interning

* fix comment style, make interning opt in

* remove Offset<T>, prefer int

* avoid creating unnecessary vtable objects

* start work on tests - do not generate builder if struct has 0 fields - add int64

* support reading structs properly

* correctly handle reading vectors of structs, dartfmt

* support structs, fix unnecessary prepares

* fix bool customizations

* undo unintentional removal of file

* docs updates, complete tutorial, bug fix for codegen

* more documentation

* Update docs, add to doxygen file

* update package structure, add samples script/code

* rearrange sample

* Tests

* Add readme for pub

* cleanup package for pub

* update docs for renamed file

* remove custom matcher, use `closeTo` instead

* remove unintentional file

* remove unintended file checkin

* use auto, move method, cleanup

* refactor to ObjectBuilders, add Builders

* Update tests, examples

* Add files missing from previous commit

* documentation and example updates

* Update LICENSE, make dartanalyzer happy, fix minor bugs, get rid of duplicate files, publish script

* fix sample for slightly different schema

* Update pubspec.yaml

dart/test/monster_test_my_game.example2_generated.dart

@@ -0,0 +1,60 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+// ignore_for_file: unused_import, unused_field, unused_local_variable
+
+library my_game.example2;
+
+import 'dart:typed_data' show Uint8List;
+import 'package:flat_buffers/flat_buffers.dart' as fb;
+
+import './monster_test_my_game_generated.dart' as my_game;
+import './monster_test_my_game.example_generated.dart' as my_game_example;
+
+class Monster {
+  Monster._(this._bc, this._bcOffset);
+  factory Monster(List<int> bytes) {
+    fb.BufferContext rootRef = new fb.BufferContext.fromBytes(bytes);
+    return reader.read(rootRef, 0);
+  }
+
+  static const fb.Reader<Monster> reader = const _MonsterReader();
+
+  final fb.BufferContext _bc;
+  final int _bcOffset;
+
+
+  @override
+  String toString() {
+    return 'Monster{}';
+  }
+}
+
+class _MonsterReader extends fb.TableReader<Monster> {
+  const _MonsterReader();
+
+  @override
+  Monster createObject(fb.BufferContext bc, int offset) => 
+    new Monster._(bc, offset);
+}
+
+class MonsterObjectBuilder extends fb.ObjectBuilder {
+
+  MonsterObjectBuilder();
+
+  /// Finish building, and store into the [fbBuilder].
+  @override
+  int finish(
+    fb.Builder fbBuilder) {
+    assert(fbBuilder != null);
+
+    fbBuilder.startTable();
+    return fbBuilder.endTable();
+  }
+
+  /// Convenience method to serialize to byte list.
+  @override
+  Uint8List toBytes([String fileIdentifier]) {
+    fb.Builder fbBuilder = new fb.Builder();
+    int offset = finish(fbBuilder);
+    return fbBuilder.finish(offset, fileIdentifier);
+  }
+}