Don't use a special var descriptor for :async_op since it can now be either stack or context alloca…

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 868ef91e882c433878702abf3d5e082c324b80e6..7a90263b866d9fd35b7c35b8068fc4499fc0e9a7 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -11470,9 +11470,6 @@ static const char* VarKindString(int kind) {
     case RawLocalVarDescriptors::kSavedCurrentContext:
       return "CurrentCtx";
       break;
-    case RawLocalVarDescriptors::kAsyncOperation:
-      return "AsyncOperation";
-      break;
     default:
       UNREACHABLE();
       return "Unknown";
@@ -11589,8 +11586,6 @@ const char* LocalVarDescriptors::KindToStr(intptr_t kind) {
       return "ContextLevel";
     case RawLocalVarDescriptors::kSavedCurrentContext:
       return "SavedCurrentContext";
-    case RawLocalVarDescriptors::kAsyncOperation:
-      return "AsyncOperation";
     default:
       UNIMPLEMENTED();
       return NULL;