Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/stack_trace/src/lazy_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.
+
+library lazy_trace;
+
+import 'frame.dart';
+import 'trace.dart';
+
+/// A thunk for lazily constructing a [Trace].
+typedef Trace TraceThunk();
+
+/// A wrapper around a [TraceThunk]. This works around issue 9579 by avoiding
+/// the conversion of native [StackTrace]s to strings until it's absolutely
+/// necessary.
+class LazyTrace implements Trace {
+  final TraceThunk _thunk;
+  Trace _inner;
+
+  LazyTrace(this._thunk);
+
+  Trace get _trace {
+    if (_inner == null) _inner = _thunk();
+    return _inner;
+  }
+
+  List<Frame> get frames => _trace.frames;
+  StackTrace get vmTrace => _trace.vmTrace;
+  Trace get terse => new LazyTrace(() => _trace.terse);
+  Trace foldFrames(bool predicate(frame)) =>
+    new LazyTrace(() => _trace.foldFrames(predicate));
+  String toString() => _trace.toString();
+
+  // Work around issue 14075.
+  set frames(_) => throw new UnimplementedError();
+}