[RN 0.29.0] Rationalize RCTBundleURLProvider

[lostintangent]

As of React Native 0.29, the generated AppDelegate.m file includes the following code to determine the JS bundle's location:

NSURL *jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];

The nice thing about this new solution, is that it handles using the packager in debug-mode and the bundled JS file in release mode (kind of like how Android already works). Therefore, devs don't need to use an #if DEBUG statement to toggle between the two.

We should think about how we expect devs to use CodePush in conjunction with this new API and/or if we need to expose a new API that effectively uses the RCTBundleURLProvider under the hood (e.g. [CodePush jsBundleURLForBundleRoot]).

I validated that CodePush works great on iOS in RN 0.29, but we need to update our docs and potentially our API in order to fully embrace the new AppDelegate.m code.

[lostintangent]

After thinking about this again, I wonder whether the simplest thing to do here is to simply update our docs to call out the following for RN v0.29.0+ apps:

NSURL *jsCodeLocation;

#ifdef DEBUG
    jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];
#else
    jsCodeLocation = [CodePush bundleURL];
#endif

The only thing that is "odd" about that, is that the RCTBundleURLProvider will also return the path to the binary if the packager isn't running. So if the app was in debug-mode, but the dev had for some reason stopped the packager, the app would use the binary-bundled bits, which isn't necessarily obvious. I think it's fine for now though.

[stovmascript]

My DEBUG build is acting strangely with this configuration, which I've made myself and thought would work. It will try to load the offline bundle - the dev menu also looks like the offline bundle should be loaded, since half of the options (live reload, etc.) are missing. But after reloading, I get a redbox error complaining that the packager server isn't running.