Fix for issue 9617, throw a snapshot write error when encountering stacktrace objects instead of an…

tests/isolate/stacktrace_message_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 'dart:isolate';
+
+main() {
+  spawnFunction(runTest).call(null);
+}
+
+runTest() {
+  port.receive((param, sendport) {
+    try {
+      throw 'sorry';
+    } catch (e, stack) {
+      try {
+        sendport.send(stack);
+        print("Stacktrace sent");
+      } catch (e) {
+        print("Stacktrace not sent");
+        sendport.send(null);
+      }
+    }
+  });
+}