How to yield in a resolver?
I'm trying to use the generator in the resolver of a graphql type? But it doesn't seem to work.

``` js
new GraphQLObjectType({
  name: 'MyType',
  fields: {
    myField: {
      type: GraphQLString,
      resolve: function *(parentValue, args, session) {
          return yield fetch("/some/api")
      }
    }
  }
});
```

The returned value is this

```
{
  "myField": ["Object generator"]
}
```

I guess its not possible to make resolvers synchronous using yields.
