Handle TypeArguments correctly

See #760 for context. We can suppress the type arguments reasonably, but handling them generally is significantly more work. In a generic function, we can return a single TypeArguments object that has the generic type parameters. This is doable if we have the function. We can ask for _runtimeType and get typeFormals. But in a frame we don't have easy access to the function, and we'd have to accumulate the type arguments from the list of all properties and separate them. It's not clear that any of the clients do much of anything with these, particularly in a stack frame context, so I think this is low priority.

Start with issue #760 for the missing context, then trace how TypeArguments are represented for generic functions and stack frames. Check how _runtimeType and typeFormals are available for functions, and review whether current clients consume this data. Done requires an agreed general handling for both function and frame contexts rather than only suppression.

Written by the indexing model from the issue text.