Have better typed code in dart:async's zone implementation

[mkustermann]

Right now completing a completer calls _Future.handleValue

  FutureOr<T> handleValue(S sourceResult) {
    return _zone.runUnary<FutureOr<T>, S>(_onValue, sourceResult);
  }

Which calls _CustomZone.runUnary:

  R runUnary<R, T>(R f(T arg), T arg) {
    var implementation = this._runUnary;
    assert(implementation != null);
    ZoneDelegate parentDelegate = _parentDelegate(implementation.zone);
    RunUnaryHandler handler = implementation.function;
    return handler(implementation.zone, parentDelegate, this, f, arg);
  }

Which accesses the _CustomZone._runUnary callback:

class _CustomZone {
  _ZoneFunction<Function> _runUnary;
  ...;
}

class _ZoneFunction<T extends Function> {
  final _Zone zone;
  final T function;
  const _ZoneFunction(this.zone, this.function);
}

Notice above that RunUnaryHandler handler = implementation.function will cause us to perform a runtime type cast against RunUnaryHandler:

typedef RunUnaryHandler = R Function<R, T>(
    Zone self, ZoneDelegate parent, Zone zone, R Function(T arg) f, T arg);

@lrhn Can we avoid these hot runtime type checks in the async/zone implementation?

[mkustermann]

/cc @mraleph

[lrhn]

Maybe!

The problem is that the correct type is ZoneFunction<R Function<R, T>(T)>, but you cannot have generic function types as type arguments (dart-lang/language#496).
So, we cannot just correct the type.

Maybe we can make a specific class, UnaryZoneFunction, with the exact same signatures as ZoneFunction<R Function<R, T>(T)> and use that instead of ZoneFunction.
We'll need to do the same for run, runBinary, bind, bindUnary, bindBinary, etc. (maybe some can be reused, I'm guessing runUnary and bindUnary will have the same signatures).

[lrhn]

Proof-of-concept code: https://dart-review.googlesource.com/c/sdk/+/141884 (completely untested yet).