Assert no VM handles are created in the zone that belongs to a ApiNativeScopes.

runtime/vm/dart_api_state.h

Index: runtime/vm/dart_api_state.h
diff --git a/runtime/vm/dart_api_state.h b/runtime/vm/dart_api_state.h
index 2565bfe6bc41b7a686dceacd75539c00d9222318..4f2935409941f9de21e87234200e771d9a31995d 100644
--- a/runtime/vm/dart_api_state.h
+++ b/runtime/vm/dart_api_state.h
@@ -53,6 +53,12 @@ class ApiZone {
   // Delete all memory associated with the zone.
   ~ApiZone() {
     Isolate* isolate = Isolate::Current();
+#if defined(DEBUG)
+    if (isolate == NULL) {
+      ASSERT(zone_.handles()->CountScopedHandles() == 0);
+      ASSERT(zone_.handles()->CountZoneHandles() == 0);
+    }
+#endif
     if ((isolate != NULL) && (isolate->current_zone() == &zone_)) {
       isolate->set_current_zone(zone_.previous_);
     }
@@ -636,7 +642,12 @@ class ApiNativeScope {
         OSThread::GetThreadLocal(Api::api_native_key_));
   }
 
-  Zone* zone() { return zone_.GetZone(); }
+  Zone* zone() {
+    Zone* result = zone_.GetZone();
+    ASSERT(result->handles()->CountScopedHandles() == 0);
+    ASSERT(result->handles()->CountZoneHandles() == 0);
+    return result;
+  }
 
  private:
   ApiZone zone_;