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

pkg/watcher/lib/src/stat.dart

Index: pkg/watcher/lib/src/stat.dart
diff --git a/pkg/watcher/lib/src/stat.dart b/pkg/watcher/lib/src/stat.dart
index d36eff3bdcbec7c2b33b23317300ece407aaa9e3..166d78988c5efa5a600fd594126166cf736b0fd8 100644
--- a/pkg/watcher/lib/src/stat.dart
+++ b/pkg/watcher/lib/src/stat.dart
@@ -7,6 +7,8 @@ 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);
@@ -29,5 +31,5 @@ Future<DateTime> getModificationTime(String path) {
     return new Future.value(_mockTimeCallback(path));
   }
 
-  return FileStat.stat(path).then((stat) => stat.modified);
+  return Chain.track(FileStat.stat(path)).then((stat) => stat.modified);
 }