Re: [whatwg/fetch] Body.json reviver (#104)

@domenic your answer is dismissing this issue purely on implementation details.

The idea that API `.json(reviver)` should exist seems appealing to me. 

How it is implemented is up to browsers. At "worse", a browser could have a C++ path with no argument and fallback to `.body().then(json => JSON.parse(json, reviver))` when there's an argument.

The point is:
- `Body.json()` is not consistent with `JSON.parse(text[, reviver])`. It's the same core function but it doesn't have the same API, which is surprising (in a bad way).
- JSON being what it is, revivers are quite useful. E.g. to replace strings with `Date`, since in 2018 automatically serializing dates is still an unsolved problem; or to create typed objects based on `$type`.

The work-around
`fetch("api/hello").then(r => r.body()).then(text => JSON.parse(text, reviver))`
is not very pretty, there's no reason that we shouldn't be able to do
`fetch("api/hello").then(r => r.json(reviver))`

Plus, _some_ browsers might be able to offer more optimized implementations?

An optimized C++ API is not as useful if it's unusable (`.body()` instead) or if you have to post-process your object graph after it's been deserialized (which moots the performance win).

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