UIP-2024 Support covariant props

[greglittlefield-wf]

Dart 1.22 recently added support for covariant overrides, which allows for sound tightening of method argument types.

This applies to fields/setters as well.

Currently, you cannot tighten the type of a prop in strong mode.

class FooProps {
  Iterable items;
}

class BarProps extends FooProps {
  @override
  List items; // Error: Field declaration FooProps.items cannot be overridden in BarProps.
}

However, with covariant, that should now be possible:

class FooProps {
  covariant Iterable items;
}

class BarProps extends FooProps {
  @override
  List items;
}

The transformer needs to be updated to generate setters with the covariant keyword when necessary.

-covariant Iterable items;
+Iterable get items => props[...];
+set items(covariant Iterable value) => props[...] = value;

Also worth noting:

In cases where the supertype authors did not foresee this need, it is still possible to tighten a parameter type by putting the covariant modifier on the overriding parameter declaration.

[rmconsole-wf]

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