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

src/flatc_main.cpp

@@ -58,6 +58,9 @@ int main(int argc, const char *argv[]) {
     { flatbuffers::GenerateJS, "-s", "--js", "JavaScript", true, nullptr,
       flatbuffers::IDLOptions::kJs,
       "Generate JavaScript code for tables/structs", flatbuffers::JSMakeRule },
+    { flatbuffers::GenerateDart, "-d", "--dart", "Dart", true, nullptr,
+      flatbuffers::IDLOptions::kDart,
+      "Generate Dart classes for tables/structs", flatbuffers::DartMakeRule },
     { flatbuffers::GenerateJS, "-T", "--ts", "TypeScript", true, nullptr,
       flatbuffers::IDLOptions::kTs,
       "Generate TypeScript code for tables/structs", flatbuffers::JSMakeRule },