[vm/concurrency] Add FLAG_enable_isolate_groups, still defaulting to true

Issue #36097

Change-Id: I3ab8f025eca164b450321abb1b7ef6d7052e7d20
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113998
Reviewed-by: Alexander Aprelev <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

runtime/lib/isolate.cc

@@ -28,6 +28,11 @@
 
 namespace dart {
 
+DEFINE_FLAG(bool,
+            enable_isolate_groups,
+            true,
+            "Whether to enable support for isolate groups.");
+
 DEFINE_NATIVE_ENTRY(CapabilityImpl_factory, 0, 1) {
   ASSERT(
       TypeArguments::CheckedHandle(zone, arguments->NativeArgAt(0)).IsNull());
@@ -152,7 +157,8 @@ class SpawnIsolateTask : public ThreadPool::Task {
     // Create a new isolate.
     char* error = nullptr;
     Isolate* isolate = nullptr;
-    if (group == nullptr || initialize_callback == nullptr) {
+    if (!FLAG_enable_isolate_groups || group == nullptr ||
+        initialize_callback == nullptr) {
       // Make a copy of the state's isolate flags and hand it to the callback.
       Dart_IsolateFlags api_flags = *(state_->isolate_flags());
       isolate = reinterpret_cast<Isolate*>((create_group_callback)(