WidgetStatesConstraint mixin

This mixin allows WidgetState enum values to be combined using the &, |, and ~ operators.

A Map with WidgetStatesConstraint objects as keys can be used in the WidgetStateProperty.fromMap constructor to resolve to one of its values, based on the first key that isSatisfiedBy the current set of states.

Example:

// This WidgetStateMap<Color?> resolves to null if no keys match.
WidgetStateProperty<Color?>.fromMap(<WidgetStatesConstraint, Color?>{
  WidgetState.error: Colors.red,
  WidgetState.hovered & WidgetState.focused: Colors.blueAccent,
  WidgetState.focused: Colors.blue,
  ~WidgetState.disabled: Colors.black,
});

// The same can be accomplished with a WidgetPropertyResolver,
// but it's more verbose:
WidgetStateProperty.resolveWith<Color?>((Set<WidgetState> states) {
  if (states.contains(WidgetState.error)) {
    return Colors.red;
  } else if (states.contains(WidgetState.hovered) && states.contains(WidgetState.focused)) {
    return Colors.blueAccent;
  } else if (states.contains(WidgetState.focused)) {
    return Colors.blue;
  } else if (!states.contains(WidgetState.disabled)) {
    return Colors.black;
  }
  return null;
});

A widget state combination can be stored in a variable, and WidgetState.any can be used for non-nullable types to ensure that there's a match:

final WidgetStatesConstraint selectedError = WidgetState.selected & WidgetState.error;

final WidgetStateProperty<Color> color = WidgetStateProperty<Color>.fromMap(
  <WidgetStatesConstraint, Color>{
    selectedError & WidgetState.hovered: Colors.redAccent,
    selectedError: Colors.red,
    WidgetState.any: Colors.black,
  },
);

// The (more verbose) WidgetPropertyResolver implementation:
final WidgetStateProperty<Color> colorResolveWith = WidgetStateProperty.resolveWith<Color>(
  (Set<WidgetState> states) {
    if (states.containsAll(<WidgetState>{WidgetState.selected, WidgetState.error})) {
      if (states.contains(WidgetState.hovered)) {
        return Colors.redAccent;
      }
      return Colors.red;
    }
    return Colors.black;
  },
);