Add stack chain support to pkg/watcher and pkg/http.

pkg/watcher/lib/src/stat.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 watcher.stat;
 
 import 'dart:async';
 import 'dart:io';
 
+import 'package:stack_trace/stack_trace.dart';
+
 /// A function that takes a file path and returns the last modified time for
 /// the file at that path.
 typedef DateTime MockTimeCallback(String path);
 
 MockTimeCallback _mockTimeCallback;
 
 /// Overrides the default behavior for accessing a file's modification time
 /// with [callback].
 ///
 /// The OS file modification time has pretty rough granularity (like a few
 /// seconds) which can make for slow tests that rely on modtime. This lets you
 /// replace it with something you control.
 void mockGetModificationTime(MockTimeCallback callback) {
   _mockTimeCallback = callback;
 }
 
 /// Gets the modification time for the file at [path].
 Future<DateTime> getModificationTime(String path) {
   if (_mockTimeCallback != null) {
     return new Future.value(_mockTimeCallback(path));
   }
 
-  return FileStat.stat(path).then((stat) => stat.modified);
+  return Chain.track(FileStat.stat(path)).then((stat) => stat.modified);
 }