Closures should not close over unnecessary variables.

[mmcdon20]

In #47972 @lrhn said there was probably already an issue for this, but I couldn't find it, so apologies if this is a duplicate.

If closures only closed over variables that are actually used inside the closure, it would make working with Isolates easier.

As of dart 2.15 Isolate.spawn can accept a closure, but will fail at runtime if one of the variables it closes over is one of the non supported message types like a ReceivePort, or UserTag. The program will fail even if the variable is never used within the closure itself.

Consider this example program.

import 'dart:isolate';

void main() async {
  final receivePort = ReceivePort();
  int three = 3;
  await Isolate.spawn(
    (SendPort port) => Isolate.exit(port, [1, 2, three]),
    receivePort.sendPort,
  );
  print(receivePort.first);
}

Which fails at runtime with the error message:

Unhandled exception:
Invalid argument(s): Illegal argument in isolate message: (object is aReceivePort)

[mraleph]

Duplicate of #36983

[mraleph]

We use #36983 to track this.