[dart] fix toString() method generated for enum value

[fawdlstty]

example fbs file:

enum Foo : ubyte {
  Bar0 = 0,
  Bar1,
  Bar2,
}

before change:

@override
String toString() {
  return 'Foo{value: $value}';
}

after change:

@override
String toString() {
  switch (value) {
    case 0: return "Bar0";
    case 1: return "Bar1";
    case 2: return "Bar2";
    default: "";
  }
}

[vaind]

The proposed format loses the information about the containing enum. What's the reasoning behind that proposal?

Also, if we should change this, i'd suggest aligning with dart Enum toString() instead:

enum Color { red, green, blue }

main() {
  test('test', () {
    expect(Color.red.toString(), 'Color.red');
  });
}

Additionally, that would seem like a good time to switch to actual enums. Do you think we'd be able to do all that's needed to construct in the scope of flatbuffers if we use enhanced enum syntax?

[vaind]

Update: I've looked into enhanced enums and it works fine, even with the default toString. I'll prep a PR.

[fawdlstty]

@vaind It seems that there will also be issues with #5467

[vaind]

Current code already handles that and throws in such a case: throw StateError('Invalid value $value for bit flag enum Abc');. Whether that's the best thing to do is a different topic.

the behavior remains unchanged after the change to enhanced enums (#8313)

[fawdlstty]

See what others think. If the new PR is merged, I will close this PR