Fix context level info in for-loops

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 c5460be9ff01b152ae9b246658cd9b9bb40edf5c..4bf3ec999cf74a8a5df01081991a6e97871525bc 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -2176,8 +2176,13 @@ void EffectGraphVisitor::VisitForNode(ForNode* node) {
     }
     Goto(loop_entry);
     exit_ = loop_entry;
+    // Note: the stack overflow check happens on the back branch that jumps
+    // to the increment instruction. The token position for the overflow
+    // check must match the position of the increment expression, so that
+    // the context level (if any) matches the that of the increment
+    // expression.
     AddInstruction(
-        new(Z) CheckStackOverflowInstr(node->token_pos(),
+        new(Z) CheckStackOverflowInstr(node->increment()->token_pos(),
                                        owner()->loop_depth()));
   }