Handle null isolate case in service tracing

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 3e9070ea40c691c0fcf4e66b47e702c41d209fbe..c7c8c5d68f49072046bda9372ce5dd62cb8690b4 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -737,10 +737,13 @@ void Service::PostEvent(const char* stream_id,
 
   if (FLAG_trace_service) {
     Isolate* isolate = Isolate::Current();
-    ASSERT(isolate != NULL);
+    const char* isolate_name = "<no current isolate>";
+    if (isolate != NULL) {
+      isolate_name = isolate->name();
+    }
     OS::Print(
         "vm-service: Pushing event of type %s to stream %s (%s)\n",
-        kind, stream_id, isolate->name());
+        kind, stream_id, isolate_name);
   }
 
   Dart_PostCObject(ServiceIsolate::Port(), &list_cobj);