Support IPv6 dev server URLs in legacy standalone RDT connection

Summary:
Changelog: [Internal]

A minimal tweak to a legacy code path for React DevTools in React Native (**NOT** Fusebox!) that enables it to work / not crash when encountering an IPv6 dev server address. See doc comment for more.

Reviewed By: hoxyq

Differential Revision: D77150288

fbshipit-source-id: c11c742aad7b83861a1242dd13c5ed2753fbdf29

packages/react-native/Libraries/Core/setUpReactDevTools.js

@@ -143,10 +143,7 @@ if (__DEV__) {
       // Get hostname from development server (packager)
       const devServer = getDevServer();
       const host = devServer.bundleLoadedFromServer
-        ? devServer.url
-            .replace(/https?:\/\//, '')
-            .replace(/\/$/, '')
-            .split(':')[0]
+        ? guessHostFromDevServerUrl(devServer.url)
         : 'localhost';
 
       // Read the optional global variable for backward compatibility.
@@ -259,3 +256,23 @@ function readReloadAndProfileConfig(
     onReloadAndProfileFlagsReset,
   };
 }
+
+/**
+ * This is a bad, no good, broken hack to get the host from a dev server URL for the purposes
+ * of connecting to the legacy React DevTools socket (for the standalone react-devtools package).
+ * It has too many bugs to list. Please don't use it in new code.
+ *
+ * The correct implementation would just be `return new URL(url).host`, but React Native does not
+ * ship with a spec-compliant `URL` class yet. Alternatively, this can be deleted when we delete
+ * `connectToWSBasedReactDevToolsFrontend`.
+ */
+function guessHostFromDevServerUrl(url: string): string {
+  const hopefullyHostAndPort = url
+    .replace(/https?:\/\//, '')
+    .replace(/\/$/, '');
+  // IPv6 addresses contain colons, so the split(':') below will return garbage.
+  if (hopefullyHostAndPort.includes(']')) {
+    return hopefullyHostAndPort.split(']')[0] + ']';
+  }
+  return hopefullyHostAndPort.split(':')[0];
+}