Correcting fetch's `json()`
Correcting fetch's `json()`

Summary:Fetch's json() should use `Promise<any>`, not `Promise<Object>`, since `any` is what `JSON.parse()` returns, and `json()` just gets text and runs `JSON.parse()` on it.
Closes https://github.com/facebook/flow/pull/1636

Differential Revision: D3154199

Pulled By: samwgoldman

fb-gh-sync-id: c2b496da9e6b3bf98961226398596a05f940f0df
fbshipit-source-id: c2b496da9e6b3bf98961226398596a05f940f0df

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