Can use @cacheControl directive with codegen

When I try and generate flow types for my schema I get an error saying:
```
Failed to generate types for src/modules/compute/service/ui/__fixtures__/query.graphql
› Error: Error initializing Apollo GraphQL project "Unnamed Project": Error: Error in "Loading schema for Unnamed Project":
› Error: Unknown directive "cacheControl".
```

My cmd to generate looks like this.
```
./node_modules/.bin/apollo client:codegen ${file}.types.js \
--localSchemaFile ${PATH_TO_MERGED_SCHEMA} \
--target flow \
--no-addTypename \
--includes ${file} \
--outputFlat \
--useFlowExactObjects \
--passthroughCustomScalars
```

It worked fine on version 1.9.2, but breaks on version 2.1.7.

Reproduce the failure with the shown `apollo client:codegen` command, using the merged schema and a query containing `@cacheControl`. Trace schema loading and directive validation for the `--target flow` path; done means codegen accepts the directive and generates the requested Flow types without the unknown-directive error.

Written by the indexing model from the issue text.