Fix crash when determining whether an async exception is handled

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index 1d947499d4e9d1baeed0a7e5ac20af1b136b1461..26113e2af4b61f522ba5ee3df745feff3efca082 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -806,6 +806,10 @@ RawObject* ActivationFrame::GetCausalStack() {
 
 
 bool ActivationFrame::HandlesException(const Instance& exc_obj) {
+  if ((kind_ == kAsyncSuspensionMarker) || (kind_ == kAsyncCausal)) {
+    // These frames are historical.
+    return false;
+  }
   intptr_t try_index = TryIndex();
   if (try_index < 0) {
     return false;