bootstrap: improve snapshot unsupported builtin warnings

- Only emit warning when the snapshot is built. In general built-ins
  loaded after the snapshot is built should work as usual.
- Clarify what the warning means

PR-URL: #50944
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/main/mksnapshot.js

@@ -19,7 +19,7 @@ const {
 const { isExperimentalSeaWarningNeeded } = internalBinding('sea');
 
 const { emitExperimentalWarning } = require('internal/util');
-
+const { emitWarningSync } = require('internal/process/warning');
 const {
   getOptionValue,
 } = require('internal/options');
@@ -29,6 +29,7 @@ const {
   namespace: {
     addSerializeCallback,
     addDeserializeCallback,
+    isBuildingSnapshot,
   },
 } = require('internal/v8/startup_snapshot');
 
@@ -119,10 +120,18 @@ function requireForUserSnapshot(id) {
     err.code = 'MODULE_NOT_FOUND';
     throw err;
   }
-  if (!supportedInUserSnapshot(normalizedId)) {
+  if (isBuildingSnapshot() && !supportedInUserSnapshot(normalizedId)) {
     if (!warnedModules.has(normalizedId)) {
-      process.emitWarning(
-        `built-in module ${id} is not yet supported in user snapshots`);
+      // Emit the warning synchronously in case we don't get to process
+      // the tick and print it before the unsupported built-in causes a
+      // crash.
+      emitWarningSync(
+        `It's not yet fully verified whether built-in module "${id}" ` +
+        'works in user snapshot builder scripts.\n' +
+        'It may still work in some cases, but in other cases certain ' +
+        'run-time states may be out-of-sync after snapshot deserialization.\n' +
+        'To request support for the module, use the Node.js issue tracker: ' +
+        'https://github.com/nodejs/node/issues');
       warnedModules.add(normalizedId);
     }
   }