`mergeFromJsonMap` message-field merge behavior is incorrect or inconsistent with other merge methods

[osa1]

When merging a message-field all merge methods (binary, proto3 JSON, merging from another message) recursively merge fields of the message being merged to the message-field.

But mergeFromJsonMap overwrites the message-field. Repro, as a test case added to protoc_plugin/test/merge_test.dart:

  test('nested message merges recursively merges message-fields', () {
    pb.Outer newMessage() => pb.Outer()
      ..inner = (pb.Inner()
        ..value = 'test'
        ..id = Int64(123));

    void checkUpdatedMessage(pb.Outer msg) {
      expect(msg.inner.value, 'test');
      expect(msg.inner.id, Int64(100));
    }

    {
      final msg = newMessage();
      msg.mergeFromBuffer(<int>[
        (4 << 3) | 2, // tag = 4, type = length delimited
        2, // length
        (2 << 3) | 0, // tag = 2, type = varint
        100, // int64 id = 100
      ]);
      checkUpdatedMessage(msg);
    }

    {
      final msg = newMessage();
      msg.mergeFromJsonMap({'4': {'2': 100}});
      checkUpdatedMessage(msg);
    }

    {
      final msg = newMessage();
      msg.mergeFromMessage(pb.Outer()..inner = (pb.Inner()..id = Int64(100)));
      checkUpdatedMessage(msg);
    }

    {
      final msg = newMessage();
      msg.mergeFromProto3Json({'inner': {'id': 100}});
      checkUpdatedMessage(msg);
    }
  });

checkUpdatedMessage after mergeFromJsonMap fails, because mergeFromJsonMap overwrites the inner field and effectively clears inner.value.

[osa1]

Given how widely used mergeFromJsonMap is, I doubt this can be fixed. We will probably have to make this a "feature" and document it.