[vm] Preserve ThreadLocal value if dart::Thread is reclaimed

dart::Thread object representing Isolate's mutator can be reclaimed when
thread is suspended, so we need to preserve thread_locals on Isolate
itself.

TEST=vm/dart/thread_local_test

Fixes https://github.com/dart-lang/sdk/issues/63408

Change-Id: I7502b9bc67a07fb2e82479d3052740516a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/504921
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

runtime/tests/vm/dart/thread_local_test.dart

@@ -0,0 +1,32 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Tests ThreadLocal.
+//
+// VMOptions=--experimental-shared-data
+
+import 'dart:isolate';
+import 'dart:_vm' show ThreadLocal;
+
+import 'package:expect/expect.dart';
+import 'package:expect/async_helper.dart';
+
+@pragma('vm:shared')
+final ThreadLocal<String> threadLocal = ThreadLocal<String>();
+
+void main() async {
+  asyncStart();
+  // Make sure that threadLocal retains its value even if underlying
+  // dart::Thread gets reclaimed and recreated.
+  final results = await List.generate(
+    64,
+    (_) => Isolate.run(() async {
+      threadLocal.value = "ok";
+      await Future.delayed(const Duration(milliseconds: 10));
+      return threadLocal.hasValue ? threadLocal.value : "fail";
+    }),
+  ).wait;
+  Expect.listEquals(['ok'], results.toSet().toList());
+  asyncEnd();
+}