Track async causal stack traces

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index fa250d4a6e1026ea105c092a5e4471a990931ee2..47deca12fa9103cfaed05c64da6135810a2fde9d 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -2279,6 +2279,20 @@ void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
   __ ret();
 }
 
+
+void Intrinsifier::ClearAsyncThreadStackTrace(Assembler* assembler) {
+  __ LoadObject(RAX, Object::null_object());
+  __ movq(Address(THR, Thread::async_stack_trace_offset()), RAX);
+  __ ret();
+}
+
+
+void Intrinsifier::SetAsyncThreadStackTrace(Assembler* assembler) {
+  __ movq(Address(THR, Thread::async_stack_trace_offset()), RAX);
+  __ LoadObject(RAX, Object::null_object());
+  __ ret();
+}
+
 #undef __
 
 }  // namespace dart