It seems there are a few methods for querying whether an Interface represents a constraint (e.g., HasTypeList, IsComparable, IsConstraint), but there don't seem to be any APIs currently for users to iterate the type list or to construct the same interface again. This is necessary to support exporting constraints that use type lists.
With the pending CL 321689, interfaces that contain constraints are not different anymore from interfaces that have embedded types. There's one new type(set), called a Union. I believe we simply need to export the explicitly declared methods and embedded elements, some of which may be unions. A union in turn is simply a list of types, together with a bit for each type indicating whether it's a "~" or not. Upon import, we should construct the internal representation lazily.
I'm going to close this issue. We have the APIs needed in types2 on the dev.typeparams, and I expect the corresponding go/types APIs will be included as part of @findleyr 's go/types API extensions proposals.