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/README.md

@@ -0,0 +1,13 @@
+# FlatBuffers for Dart
+
+This package is used to read and write FlatBuffer files in Dart.
+
+Most consumers will want to use the [`flatc`](https://github.com/google/flatbuffers)
+compiler to generate Dart code from a FlatBuffers IDL schema.  For example, the
+`monster_my_game.sample_generated.dart` was generated with `flatc` from
+`monster.fbs` in the example folder. The generated classes can be used to read
+or write binary files that are interoperable with other languages and platforms
+supported by FlatBuffers, as illustrated in the `example.dart` in the
+examples folder.
+
+Additional documentation and examples are available [at the FlatBuffers site](https://google.github.io/flatbuffers/index.html)