False positive from unused variables validator when query variable is only used in object literal passed to @arguments

If I have a query like this:
```graphql
query TestQuery($foo: String!) {
...TestFragment @arguments(input: { foo: $foo })
}
```
I get an error like: "Variable `$foo` is never used in operation `TestQuery`"

However if I use that variable in another fragment without the object literal, I get no error and it compiles happily.

```graphql
query TestQuery($foo: String!) {
...TestFragment @arguments(input: { foo: $foo })
...TestTwoFragment @arguments(foo: $foo)
}
```

Is this a bug in the unused variables validator?