Migrated test block 27 to Dart 2.0.

tests/corelib_strong/stacktrace_fromstring_test.dart

-// Copyright (c) 2015, 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";
-
-void main() {
-  StackTrace stack;
-  try {
-    throw 0;
-  } catch (e, s) {
-    stack = s;
-  }
-  var string = "$stack";
-  StackTrace stringTrace = new StackTrace.fromString(string);
-  Expect.isTrue(stringTrace is StackTrace);
-  Expect.equals(stack.toString(), stringTrace.toString());
-
-  string = "some random string, nothing like a StackTrace";
-  stringTrace = new StackTrace.fromString(string);
-  Expect.isTrue(stringTrace is StackTrace);
-  Expect.equals(string, stringTrace.toString());
-
-  // Use stacktrace asynchronously.
-  asyncStart();
-  var c = new Completer();
-  c.completeError(0, stringTrace);
-  c.future.then((v) {
-    throw "Unexpected value: $v";
-  }, onError: (e, s) {
-    Expect.equals(string, s.toString());
-  }).then((_) {
-    var c = new StreamController();
-    c.stream.listen((v) {
-      throw "Unexpected value: $v";
-    }, onError: (e, s) {
-      Expect.equals(string, s.toString());
-      asyncEnd();
-    });
-    c.addError(0, stringTrace);
-    c.close();
-  });
-}