[vm] Avoid deadlock in Isolate.debugName.

TEST=ci
Bug: https://github.com/dart-lang/sdk/issues/62237
Change-Id: I57bfcc341e34aa9e873092e0f7ebf44bc2f95cfc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/476143
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/tests/vm/dart/isolates/many_debug_name_test.dart

@@ -0,0 +1,24 @@
+// Copyright (c) 2026, 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.
+
+import 'dart:isolate';
+
+child(replyPort) {
+  for (var i = 0; i < 10; i++) {
+    replyPort.send(Isolate.current.debugName);
+  }
+}
+
+main() async {
+  var pending = 0;
+  var port = new RawReceivePort();
+  port.handler = (_) {
+    pending--;
+    if (pending == 0) port.close();
+  };
+  for (var i = 0; i < 20; i++) {
+    pending += 10;
+    Isolate.spawn(child, port.sendPort);
+  }
+}

runtime/vm/isolate.cc

@@ -3617,9 +3617,6 @@ void Isolate::PauseEventHandler() {
 #endif  // !defined(PRODUCT)
 
 void Isolate::VisitIsolates(IsolateVisitor* visitor) {
-  if (visitor == nullptr) {
-    return;
-  }
   IsolateGroup::ForEach([&](IsolateGroup* group) {
     group->ForEachIsolate(
         [&](Isolate* isolate) { visitor->VisitIsolate(isolate); });
@@ -3635,7 +3632,6 @@ intptr_t Isolate::IsolateListLength() {
 }
 
 std::unique_ptr<char[]> Isolate::LookupIsolateNameByPort(Dart_Port port) {
-  MonitorLocker ml(isolate_creation_monitor_);
   std::unique_ptr<char[]> result;
   IsolateGroup::ForEach([&](IsolateGroup* group) {
     group->ForEachIsolate([&](Isolate* isolate) {