Track async causal stack traces

runtime/vm/stub_code_dbc.cc

Index: runtime/vm/stub_code_dbc.cc
diff --git a/runtime/vm/stub_code_dbc.cc b/runtime/vm/stub_code_dbc.cc
index ef4118222cf63b7b2f550aa8d57011dbd022f8a0..545a70ef7a62d2355958e0b604eea7a27e1e2acc 100644
--- a/runtime/vm/stub_code_dbc.cc
+++ b/runtime/vm/stub_code_dbc.cc
@@ -95,6 +95,11 @@ void StubCode::GenerateFrameAwaitingMaterializationStub(Assembler* assembler) {
 }
 
 
+void StubCode::GenerateAsynchronousGapMarkerStub(Assembler* assembler) {
+  __ Trap();
+}
+
+
 // Print the stop message.
 DEFINE_LEAF_RUNTIME_ENTRY(void, PrintStopMessage, 1, const char* message) {
   OS::Print("Stop message: %s\n", message);