Unused imports

Hi!
I faced ts checking issue. Generated ts file have unused imports in my case.
In my frontend app i use component that can handle union of types (declared by fragment). And my GQL server have query that returns single one of this types.
I wondered my single type query with fragment on union type as result (like [this one](https://github.com/apollographql/apollo-tooling/pull/1872/commits/3d1eb164e460cc95fe5633a5f8c89e50ec198448#diff-432141440860f4fb4cc16a73e7850031R641-R643) ) is valid and working. But generated type file is not valid any more. Ts checker reports about unused imports from globalTypes. They are needed for other types in union, but generated file does not contain it.

Start with the generated TypeScript file and its globalTypes imports, using the linked Apollo example to reproduce a fragment on a union returned by a single-type query. Run the TypeScript checker against that generated output; done means the generated file has no unused-import errors while retaining the types needed by the union.

Written by the indexing model from the issue text.