PbList.toFrozenPbList does not freeze the list in-place, allows modifying frozen messages

[osa1]

Discovered this while reading the code.

syntax = "proto3";

message Test {
  repeated int32 ints = 1;
}

import '../test.pb.dart';

void main() {
  Test test1 = Test.create();
  test1.ints.add(123);

  var ints = test1.ints;

  test1.freeze();
  ints.add(456);

  print(test1);
}

Output:

ints: 123
ints: 456

Expected output: it should fail with an exception.

PbList should implement freezing the same way PbMap implements it: set a bool flag in PbList, mark elements of the list as frozen.