Downloading a local .graphql schema file does not honor type extensions

Apollo server is [supposed to support](https://www.apollographql.com/docs/graphql-tools/generate-schema.html#extend-types) types that are defined in SDL using the `extend` syntax. However, when the same local SDL file that is downloaded using the apollo service:download command, those type extensions are not reflected in the updated schema.json.

Related unresolved (but closed) issue: #210. Opening a new issue to avoid confusion with old version of tooling.

Start with the apollo service:download command and trace how a local SDL file containing extend definitions becomes schema.json. Compare the downloaded schema with the original type extensions; done means those extensions are represented in the generated schema.json.

Written by the indexing model from the issue text.