Add test that makes sure print deals with broken toStrings.

tests/lib/async/print_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.
+
+// This test tests a core-library property, but uses zones to do this.
+// Verifies that print(x) works with bad toString methods.
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+
+class A {
+  toString() {
+    if (false
+          || true  /// 01: runtime error
+        ) {
+      return 499;
+    } else {
+      return "ok";
+    }
+  }
+}
+
+void interceptedPrint(self, parent, zone, message) {
+  Expect.isTrue(message is String);
+}
+
+main() {
+  runZoned(() { print(new A()); },
+           zoneSpecification: new ZoneSpecification(print: interceptedPrint));
+}