Adjust the column number where a StackOverflow error is reported.

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 959b4101de2cb160970d48c8170208d5004be4c9..98dabb2adce53b575c996bfb3c0ef0e242e1a282 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -3881,8 +3881,12 @@ void EffectGraphVisitor::VisitSequenceNode(SequenceNode* node) {
     // if we inline or not.
     if (!function.IsImplicitGetterFunction() &&
         !function.IsImplicitSetterFunction()) {
+      // We want the stack overlow error to be reported at the opening '{' or
+      // at the '=>' location. So, we get the sequence node corresponding to the
+      // body inside |node| and use its token position.
+      ASSERT(node->length() > 0);
       CheckStackOverflowInstr* check = new (Z) CheckStackOverflowInstr(
-          node->token_pos(), 0, owner()->GetNextDeoptId());
+          node->NodeAt(0)->token_pos(), 0, owner()->GetNextDeoptId());
       // If we are inlining don't actually attach the stack check. We must still
       // create the stack check in order to allocate a deopt id.
       if (!owner()->IsInlining()) {