Set dartAsyncMarker the same as asyncMarker unless explicitly set

tests/language/vm/causal_async_exception_stack_test.dart

Index: tests/language/vm/causal_async_exception_stack_test.dart
diff --git a/tests/language/vm/causal_async_exception_stack_test.dart b/tests/language/vm/causal_async_exception_stack_test.dart
index d08ae9de5f29fe8cf8bc17dbfc9c0d82154c2b87..d9110d5d60a79a62f96401b978e865e9d3fbab7b 100644
--- a/tests/language/vm/causal_async_exception_stack_test.dart
+++ b/tests/language/vm/causal_async_exception_stack_test.dart
@@ -27,6 +27,7 @@ main() async {
   // Test async and async*.
   try {
     await foo();
+    fail("Did not throw");
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([
         'thrower', '.dart:8',
@@ -72,4 +73,4 @@ main() async {
            'main', '.dart:67',
            ]));
   }
-}
+}