Add additional fallbacks when self is undefined

[tqc]

This extends the solution in #253 for React Native compatibility.

The existing this fallback is inadequate because in strict mode this is undefined outside of class methods rather than defaulting to the global object.

[mislav]

I have no experience with React Native, but their documentation suggests that fetch is available natively. If this is true, why is the polyfill needed?

[tqc]

@mislav The polyfill itself may not be needed, but libraries that support multiple platforms (including react itself) can't know that, and without self the polyfill crashes at the point where you check if it is needed.

[tqc]

@mislav incidentally, I don't think that documentation implies fetch is available natively - just that react wraps this polyfill so the app developer doesnt have to think about it.

[psullivan6]

Related to this conversation:
Using this polyfill with rollup logs the following warning during the build:

node_modules/whatwg-fetch/fetch.js (461:40) The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten

which means the final line of the script is compiled into:
})(typeof self !== 'undefined' ? self : undefined); where undefined replaced this.

---

This is using the ES6 import 'whatwg-fetch'; statement.

[AntonioRedondo]

Any idea when is this gonna be merged? We're trying to use isomorphic-fetch server-side bundled with Rollup. And the issue that this pull request is trying to solve is a blocker.

[mislav]

We now generate a UMD build with Rollup: #616. Let us know if this build works with React Native.

The main fetch.js file is an ES5 file (like before) but with added ES6 exports. This makes it compatible with Rollup, and is referred to by module keyword from package.json. This should make the library work better with packagers.