Is this a bug report?

Yes (Mostly)

Have you read the Contributing Guidelines?

Yes

Environment

1. react-native -v: 0.48.3
2. node -v: v8.4.0
3. npm -v: 5.3.0
4. yarn --version: 0.23.4

• Target Platform: iOS11

• Development Operating System: macOS 10.12.6 (16G29)

• Build tools: XCode 9 beta

Steps to Reproduce

1. Want to automate the RCTWebSocketExecutor react-devtools host detection.
2. sed -i '' it all the way... Anyways, I'm using ngrok, so, it's a bit bothersome.
3. Well, if RCTBundleURLProvider already has guessPackagerHost method which uses ip.txt file in the main bundle, we could use that to pass the hostname for the RCTWebSocketExecutor.

Expected Behavior

Less regex's and manual RCTWebSocketExecutor patching.

Actual Behavior

Official documentation states explicitly to patch the RCTWebSocketExecutor.m file which isn't a great way of handling such things.