[Flow] __typename not marked a read-only

**Describe the bug**

**To Reproduce**
Steps to reproduce the behavior:

https://codesandbox.io/s/wizardly-moser-nsrl1?file=/types.js

1. My GraphQL schema:

Any schema

2. My GraphQL operations:

No operations.

3. My `codegen.yml` config file:

```
schema: schema.graphql
documents: document.graphql
generates:
types.js:
plugins:
- flow
- flow-operations
config:
useFlowReadOnlyTypes: true
```

**Expected behavior**
__typename should be read-only