Ensure unreferenced types that implement reference interfaces are detected

[dschafer]

Ensures that if a type isn't referenced directly, but implements an interface that is references, that type is included in the schema, with a unit test to verify.

[leebyron]

This is O(N^2) since you need to filter down the AST and produceTypeDef for the implementing types for every interface you encounter, (so really O(I*N))

Here's how I did it in https://github.com/graphql/graphql-js/blob/master/src/utilities/buildClientSchema.js#L302

On a side note, you may consider comparing this file to that file to see if there are any other gaps

[dschafer]

Good call. Changed it to iterate through all types; now the produceTypeMap for the query will just hit cache (where before, it was actually constructing the types).

[leebyron]

wonderful. I love that it ended up being a one-liner and the bulk of this is tests :) land it