[vm/shared] Ensure DRT_ResumeFrame can run without isolate.

TEST=shared_resume_test

Change-Id: I06be289f538f11df3592b4e726c4a47ec4d8965d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443360
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/tests/vm/dart/shared_resume_test.dart

@@ -0,0 +1,36 @@
+// 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.
+//
+// Verify that ResumeFrame runtime call entry doesn't crash without isolates.
+//
+// VMOptions=--experimental-shared-data
+//
+
+import 'package:dart_internal/isolate_group.dart' show IsolateGroup;
+import 'package:expect/expect.dart';
+
+Iterable<int> foo() sync* {
+  yield 1;
+  throw 42;
+}
+
+Iterable<int> bar() sync* {
+  yield* foo();
+}
+
+@pragma("vm:shared")
+int caught = 0;
+
+main() async {
+  IsolateGroup.runSync(() {
+    final iterator = bar().iterator;
+    iterator.moveNext();
+    try {
+      iterator.moveNext();
+    } on int catch (e) {
+      caught = e;
+    }
+  });
+  Expect.equals(42, caught);
+}

runtime/vm/runtime_entry.cc

@@ -4209,8 +4209,10 @@ DEFINE_RUNTIME_ENTRY(ResumeFrame, 2) {
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
 #if !defined(PRODUCT)
-  if (isolate->has_resumption_breakpoints()) {
-    isolate->debugger()->ResumptionBreakpoint();
+  if (isolate != nullptr) {
+    if (isolate->has_resumption_breakpoints()) {
+      isolate->debugger()->ResumptionBreakpoint();
+    }
   }
 #endif