Implement safepointing of threads

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 35be737034cc8e4dad41d78a6a61baa1475a0c90..8b8f6f21a5e497210fb22f3b64013640d711efca 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -142,6 +142,8 @@ bool Service::ListenStream(const char* stream_id) {
     }
   }
   if (stream_listen_callback_) {
+    Thread* T = Thread::Current();
+    TransitionVMToNative transition(T);
     return (*stream_listen_callback_)(stream_id);
   }
   return false;
@@ -162,12 +164,15 @@ void Service::CancelStream(const char* stream_id) {
     }
   }
   if (stream_cancel_callback_) {
+    Thread* T = Thread::Current();
+    TransitionVMToNative transition(T);
     return (*stream_cancel_callback_)(stream_id);
   }
 }
 
 RawObject* Service::RequestAssets() {
   Thread* T = Thread::Current();
+  TransitionVMToNative transition(T);
   Api::Scope api_scope(T);
   if (get_service_assets_callback_ == NULL) {
     return Object::null();