Fix capturing variables in optimized compilations

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index b9685854b9f24866daf0a5b243c1575c35c5d6d2..108d4cebf1cc1118980a5e4d17bf3d300f323a16 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -2419,6 +2419,7 @@ void EffectGraphVisitor::VisitClosureNode(ClosureNode* node) {
   // compiler.  If it was not, set it here.
   if (function.context_scope() == ContextScope::null()) {
     ASSERT(!is_implicit);
+    ASSERT(node->scope() != NULL);
     const ContextScope& context_scope = ContextScope::ZoneHandle(
         Z, node->scope()->PreserveOuterScope(owner()->context_level()));
     ASSERT(!function.HasCode());