How to use MockStore with interface/union types

[jasnross]

I have a question about how to configure MockStore and addMocksToSchema to handle interface/union types.

For example, given the following schema:

    type Query {
      user(id: ID!): User
    }

    interface User {
      # In our case ID will always be unique across all implementing types
      id: ID!
      name: String!
    }

    type Teacher implements User {
      id: ID!
      name: String!
      classSize: Int!
    }

    type Student implements User {
      id: ID!
      name: String!
      grade: Int!
    }

... if we want to set some specific data for a record that belongs to an interface/union we can do:

store.set("Student", "student-1", {
  __typename: "Student",
  grade: 12
});

... but how do we get it back out in a resolver for the interface type User?

        user: (_parent, { id }) => {
          // Results in: Abstract type \"User\" was resolve to a non-object type \"User\"."
          // Not sure how to tell what type it should be?
          return store.get("User", id);
        }

Here is a codesandbox with an example as well: https://codesandbox.io/s/friendly-wave-5or4h?file=/src/index.js

Is there any way to configure MockStore so that it understands the relationship between union/interface types and can help resolve the correct type?