Enable causal stacktrace in kernel

runtime/vm/kernel.h

Index: runtime/vm/kernel.h
diff --git a/runtime/vm/kernel.h b/runtime/vm/kernel.h
index b3450f6f8f850bf37ab791a211fbd1ed7912a299..d5b07359b32c314fbf354c4f2c6f6e318ec602b6 100644
--- a/runtime/vm/kernel.h
+++ b/runtime/vm/kernel.h
@@ -900,7 +900,7 @@ class FunctionNode : public TreeNode {
   virtual void VisitChildren(Visitor* visitor);
 
   AsyncMarker async_marker() { return async_marker_; }
-  bool debuggable() { return debuggable_; }
+  AsyncMarker original_async_marker() { return original_async_marker_; }
   TypeParameterList& type_parameters() { return type_parameters_; }
   int required_parameter_count() { return required_parameter_count_; }
   List<VariableDeclaration>& positional_parameters() {
@@ -919,7 +919,7 @@ class FunctionNode : public TreeNode {
         end_position_(TokenPosition::kNoSource) {}
 
   AsyncMarker async_marker_;
-  bool debuggable_;
+  AsyncMarker original_async_marker_;
   TypeParameterList type_parameters_;
   int required_parameter_count_;
   List<VariableDeclaration> positional_parameters_;