Force reload at split points when Metro restarts

Summary:
In https://fb.workplace.com/groups/rn.support/permalink/3132785890103338/?comment_id=3132873900094537 we identified that when Metro restarts but the app keeps the main bundle alive, the next request for a split bundle will always produce an invalid bundle. This is because we rely on module ids being stable while Metro is running and when Metro is restarted, those module ids will likely be assigned to different modules.

The fix applied here simply forces a full reload when downloading (registering) a new split bundle if the connection to Metro disappeared.

Another way to mitigate this is to use stable module ids. That's a more involved change in Metro that I'm not planning on making right now.

Changelog: [Internal]

Reviewed By: rickhanlonii

Differential Revision: D19235555

fbshipit-source-id: b8ff10590655952a2af1379bbf4e930f9eb95c1b

Libraries/Utilities/HMRClient.js

@@ -10,6 +10,7 @@
 
 'use strict';
 
+const DevSettings = require('./DevSettings');
 const Platform = require('./Platform');
 const invariant = require('invariant');
 
@@ -274,6 +275,11 @@ function setHMRUnavailableReason(reason) {
 }
 
 function registerBundleEntryPoints(client) {
+  if (hmrUnavailableReason) {
+    DevSettings.reload('Bundle Splitting – Metro disconnected');
+    return;
+  }
+
   if (pendingEntryPoints.length > 0) {
     client.send(
       JSON.stringify({