Enable concurrent optimization test after migrating scopes to Thread*

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 467156009f86e612629f658709e393a61ceb7ee9..e5cd485be6968f8b33a0249711c33d859e719369 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -236,8 +236,9 @@ Isolate* Dart::CreateIsolate(const char* name_prefix,
 
 RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
   // Initialize the new isolate.
-  Isolate* isolate = Isolate::Current();
-  TIMERSCOPE(isolate, time_isolate_initialization);
+  Thread* thread = Thread::Current();
+  Isolate* isolate = thread->isolate();
+  TIMERSCOPE(thread, time_isolate_initialization);
   TimelineDurationScope tds(isolate,
                             isolate->GetIsolateStream(),
                             "InitializeIsolate");