Test for kernel async stacktrace bug

tests/language/vm/async_await_catch_stacktrace_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:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+void main() {
+  x() async {
+    print("Starting!");
+    try {
+      await runAsync();
+    } catch (e, st) {
+      print("Got exception and stacktrace:");
+      var stText = st.toString();
+      print(e);
+      print(stText);
+      // Stacktrace should be something like
+      // #0      runAsync.<runAsync_async_body> (this file)
+      // #1      Future.Future.microtask.<anonymous closure> (dart:async/future.dart:184)
+      // #2      _microtaskLoop (dart:async/schedule_microtask.dart:41)
+      // #3      _startMicrotaskLoop (dart:async/schedule_microtask.dart:50)
+      // #4      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
+      // #5      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)
+
+      // if exception and stacktrace is rethrown correctly, NOT
+
+      // #0      main.<anonymous closure>.async_op (this file)
+      // #1      _asyncErrorWrapperHelper.<anonymous closure> (dart:async:134:33)
+      // #2      _RootZone.runBinary (dart:async/zone.dart:1410:54)
+      // #3      _FutureListener.handleError (dart:async/future_impl.dart:146:20)
+      // #4      _Future._propagateToListeners.handleError (dart:async/future_impl.dart:649:47)
+      // #5      _Future._propagateToListeners (dart:async/future_impl.dart:671:13)
+      // #6      _Future._completeError (dart:async/future_impl.dart:485:5)
+      // #7      _SyncCompleter._completeError (dart:async/future_impl.dart:56:12)
+      // #8      _Completer.completeError (dart:async/future_impl.dart:27:5)
+      // #9      runAsync.async_op (this file)
+      // #10     Future.Future.microtask.<anonymous closure> (dart:async/future.dart:184:26)
+      // #11     _microtaskLoop (dart:async/schedule_microtask.dart:41:5)
+      // #12     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
+      // #13     _runPendingImmediateCallback (dart:isolate:1054:5)
+      // #14     _RawReceivePortImpl._handleMessage (dart:isolate:1104:5)
+
+      Expect.isFalse(stText.contains("propagateToListeners"));
+      Expect.isFalse(stText.contains("_completeError"));
+      Expect.isFalse(stText.contains("main"));
+    }
+    print("Ending!");
+  }
+
+  asyncStart();
+  x().then((_) => asyncEnd());
+}
+
+runAsync() async {
+  throw 'oh no!';
+}