Migrate async tests to strong

tests/lib_strong/async/stack_trace25_test.dart

+// Copyright (c) 2013, 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';
+import 'dart:async';
+
+StackTrace captureStackTrace() {
+  try {
+    throw 0;
+  } catch (e, st) {
+    return st;
+  }
+}
+
+main() {
+  StackTrace trace = captureStackTrace();
+  var controller;
+  controller = new StreamController(onListen: () {
+    controller.addError("error", trace);
+    controller.close();
+  });
+  asyncStart();
+  var iterator = new StreamIterator(controller.stream);
+  var future = iterator.moveNext();
+  future.then((_) { throw "unreachable"; },
+    onError: (e, st) {
+      Expect.equals("error", e);
+      Expect.identical(trace, st);
+      asyncEnd();
+    });
+}