Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/stack_trace/stack_trace.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.
+
+/**
+ * Stack trace generation and parsing.
+ *
+ * ## Installing ##
+ *
+ * Use [pub][] to install this package. Add the following to your `pubspec.yaml`
+ * file.
+ *
+ *     dependencies:
+ *       stack_trace: any
+ *
+ * Then run `pub install`.
+ *
+ * For more information, see the
+ * [stack_trace package on pub.dartlang.org][pkg].
+ *
+ * [pub]: http://pub.dartlang.org
+ * [pkg]: http://pub.dartlang.org/packages/stack_trace
+ */
+library stack_trace;
+
+export 'src/trace.dart';
+export 'src/frame.dart';
+export 'src/chain.dart';