Freezing a message does not freeze bytes fields, allows modifying the message after the freeze

[osa1]

We represent bytes fields as Uint8List, which doesn't support freezing. Repro:

syntax = "proto3";

message M1 {
  bytes b = 1;
}

void main() {
  M1 m = M1();
  m.b = <int>[0];
  m.freeze();
  m.b.add(1);
  print(m);
}