Tweak debugger safe point generation

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 44844)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -3436,10 +3436,11 @@
 void EffectGraphVisitor::VisitStoreLocalNode(StoreLocalNode* node) {
   // If the right hand side is an expression that does not contain
   // a safe point for the debugger to stop, add an explicit stub
-  // call. Exception: don't do this when assigning to internal variables,
-  // or for generated code that has no source position.
+  // call. Exception: don't do this when assigning to or from internal
+  // variables, or for generated code that has no source position.
   if ((node->value()->IsLiteralNode() ||
-      node->value()->IsLoadLocalNode() ||
+      (node->value()->IsLoadLocalNode() &&
+          !node->value()->AsLoadLocalNode()->local().IsInternal()) ||
       node->value()->IsClosureNode()) &&
       !node->local().IsInternal() &&
       (node->token_pos() != Scanner::kNoSourcePos)) {