Add an empty unmodifiable map implementation

[donny-dont]

Adds an EmptyUnmodifiableMap implementation which provides a const constructor.

The Mixin classes within unmodifiable_wrappers.dart were changed from abstract class to mixin to modernize the code. This allowed the EmptyUnmodifiableMap to have a const constructor without also defining a const constructor in UnmodifiableMapMixin.

[donny-dont]

@kevmoo @natebosch I think this would help simplify some of the shelf stuff around context and headers.

class Context extends UnmodifiableMapView<String, Object> {
  factory Context(Map<String, Object> from) {
    if (from is Context) return from;
    if (from.isEmpty) return Context.empty();
    return Context._(from);
  }
  const factory Context.empty() = _EmptyContext;
  Context._(Map<String, Object> from) : super(from);
}

class _EmptyContext extends EmptyUnmodifiableMap<String, Object>
    implements Context {
  const _EmptyContext();
}

[kevmoo]

Ooo!

Tests?
Changelog entry?
Version bump?

[donny-dont]

Sure! Wanted to make sure that the changes were acceptable before writing tests.

Version would be 1.15.0-nullsafety.6? I'm assuming after null safety lands the package is being bumped to 2.0 right?

[kevmoo]

Let @natebosch chime in before you do any more...

[natebosch]

Could we skip the extra class and use const {}?

class Context extends UnmodifiableMapView<String, Object> {
  factory Context(Map<String, Object> from) {
    if (from is Context) return from;
    if (from.isEmpty) return empty;
    return Context._(from);
  }
  static final empty = Context._(const {});
  Context._(Map<String, Object> from) : super(from);
}

Or, if we make the UnmodifiableMap constructor const it opens up options to get you closer to the pattern you had if you strongly prefer that:

class _EmptyContext extends UnmodifiableMapView<String, Object>
    implements Context {
  const _EmptyContext(): super(const {});
}

or without the empty class, but keeping the Context.empty() as a constructor and not singleton (let's imagine you need to handle generics even though this example doesn't)

class Context extends UnmodifiableMapView<String, Object> {
  factory Context(Map<String, Object> from) {
    if (from is Context) return from;
    if (from.isEmpty) return Context.empty();
    return Context._(from);
  }
  const Context.empty() : this._(const {});
  const Context._(Map<String, Object> from) : super(from);
}

I don't think I see a reason we'd need this class in this library. I'd be happy to explore whether we can make the existing constructor const.

[natebosch]

I'm assuming after null safety lands the package is being bumped to 2.0 right?

No, this and a few other very core packages are migrating to null safety without a major version bump because an early choice by the Flutter framework makes it infeasible to do breaking releases.

[mit-mit]

Closing as assumed stale