Keep track of stack base whenever Dart code is invoked.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index ad90cd3c5dbfb2c8044aeeca3b22eeb1835c11c1..a1564d215404ee04e966577e78a1dee88cf4d247 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -408,7 +408,7 @@ Isolate* Service::GetServiceIsolate(void* callback_data) {
   if (isolate == NULL) {
     return NULL;
   }
-  Isolate::SetCurrent(isolate);
+  StartIsolateScope isolate_scope(isolate);
   {
     // Install the dart:vmservice library.
     StackZone zone(isolate);
@@ -473,7 +473,6 @@ Isolate* Service::GetServiceIsolate(void* callback_data) {
     RegisterRunningIsolatesVisitor register_isolates(isolate);
     Isolate::VisitIsolates(&register_isolates);
   }
-  Isolate::SetCurrent(NULL);
   service_isolate_ = reinterpret_cast<Isolate*>(isolate);
   return service_isolate_;
 }