Dart2js workaround; custom toString()
- Generated code ignores more lints.
- Add a workaround so "polymorphism" features can be used with dart2js.
See example/lib/polymorphism.dart. - Deal explicitly with the user defining their own operator==, hashCode
and/or toString(). Previously, they would just not work. Now, custom
operator== and hashCode are forbidden at compile time, but custom
toString() is supported.