VM over-captures variables in closures

[lrhn]

This problem has been significantly improved, but still hits some very likely code patterns.
Example:

import "dart:isolate";
void main() {
  var r = RawReceivePort();
  r.handler = (v) {
    print(v);
    r.close();
  };
  var p = r.sendPort;
  Isolate.run(() => p.send(42));
}

The closure only references p, a variable containing a SendPort, which should be sendable to any other isolate.

When running, you get the errror:

Invalid argument(s): Illegal argument in isolate message: (object is a ReceivePort)
#0      Isolate._spawnFunction (dart:isolate-patch/isolate_patch.dart:399:25)
...

which shows that the closure remembers the r variable, for no apparent reason. There is no reference to r in the function expression, or any time later. I'm assuming the earlier handler closure is what triggers the problem.
If I remove the r.close from the handler, the error goes away (but I also don't get to close the port).

This is a very likely problem, my first attempt to write an example of using Isolate.run to send multiple results was code precisely like this, and I had to introduce a helper function to work around it:

Future<void> runRemote(SendPort port) => Isolate.run(() { ... p.send ...});

which moves the closure out of the scope of the RawReceivePort declaration.

[mraleph]

Duplicate of #36983