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Pull #295

travis-ci

Vendor whatwg-fetch

Pretender uses the `whatwg-fetch` package in order to intercept `fetch` calls. However, Pretender relies implicitly on `whatwg-fetch` internals to get the `fetch` intercepting behavior working correctly.

This starting causing some problems in some systems where the `whatwg-fetch` package was swapped or removed altogether, since the package is indeed a polyfill and the system determined the behavior was present and thus the polyfill wasn't needed. However, because Pretender actually relied not only on the polyfill behavior but also internal implementation details, Pretender would break in these systems.

More details: The `fetch` implementation that `whatwg-fetch` provides actually delegates to `window.XMLHTTPRequest` under the hood. Since Pretender _also_ monkey patches `window.XMLHTTPRequest`, it is able to intercept `fetch` calls equally as well. But it's only due to those internals – due to the fact that the `whatwg-fetch` polyfill happens to use XMLHTTPRequest under the hood. This is not part of `whatwg-fetch`'s public API, even though it is unlikely the change.

Since Pretender has been relying on observable but undocumented behavior of `whatwg-fetch`, this commit vendors the package, ensuring that `whatwg-fetch` can no longer be treated as an external dependency and hoisted or replaced.

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