Set dartAsyncMarker the same as asyncMarker unless explicitly set

tests/language/vm/causal_async_exception_stack_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:unittest/unittest.dart';
 
 thrower() async {
   throw 'oops';
 }
 
 number() async {
   return 4;
 }
 
 generator() async* {
   yield await number();
   yield await thrower();
 }
 
 foo() async {
   await for (var i in generator()) {
     print(i);
   }
 }
 
 main() async {
   // Test async and async*.
   try {
     await foo();
+    fail("Did not throw");
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([
         'thrower', '.dart:8',
         '<asynchronous suspension>',
         'generator', '.dart:17',
         '<asynchronous suspension>',
         'foo', '.dart:21',
         '<asynchronous suspension>',
         'main',
         ]));
@@ skipping 25 matching lines @@
   // Test for correct linkage.
   try {
     await thrower();
   } catch(e, st) {
     expect(st.toString(), stringContainsInOrder([
            'thrower', '.dart:8',
            '<asynchronous suspension>',
            'main', '.dart:67',
            ]));
   }
-}
+}