#172,
1, 10: I'd suggest to focus just on D6 here.
2, 4, 5: Done
3, 11: added descriptions
7: No, this is not defined in the base class. It has to be implemented by concrete classes.
8: This is copied from the already existing TaxonomyTermReference plugin. But you're right. I am thinking we should deal with that in a follow-up.
12: Done, but I think we can improve the naming of the method.
13: It's not changed here. I think the reason is that instances were just called fields in D6?
14: Fixed
15: Actually, this plugin is only for D6. There is a "d7_field_settings" for D7.
16: See 15.
17, 20: There are lots of other assertIdentical in the test. I think we should do this in a follow-up. I recall you raised this in another issue as well. We could combine this for all of them.
18, 19: Fixed
22: Fixed
23, 25: What should the default case do?
24: The only common base class here is CckFieldPluginBase and it doesn't make sense there. Could we inherit the D7 version from D6 like we do elsewhere?
This patch adds a lot of features to ReferenceBase for field migrations. I think some of these would make sense for D7 as well. Follow-up?
I also realize I skipped a couple of points I was not very clear on. There were lot of changes already, so we could look at them again after tests and review.