[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: "";
  }
}