Add some additional debugging prints to the mac os watcher.

pkg/watcher/lib/src/directory_watcher/mac_os.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.directory_watcher.mac_os;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 91 matching lines @@
         for (var file in _files.toSet()) {
           print("[$_id]   ${p.relative(file, from: directory)}");
         }
       }
       _readyCompleter.complete();
     });
   }
 
   void close() {
     if (MacOSDirectoryWatcher.logDebugInfo) {
-      print("[$_id] watcher is closed");
+      print("[$_id] watcher is closed\n${new Chain.current().terse}");
     }
     if (_watchSubscription != null) _watchSubscription.cancel();
     if (_initialListSubscription != null) _initialListSubscription.cancel();
     if (_listSubscription != null) _listSubscription.cancel();
     _watchSubscription = null;
     _initialListSubscription = null;
     _listSubscription = null;
     _eventsController.close();
   }
 
@@ skipping 338 matching lines @@
 
     if (MacOSDirectoryWatcher.logDebugInfo) {
       print("[$_id] emitting $type ${p.relative(path, from: directory)}");
     }
 
     _eventsController.add(new WatchEvent(type, path));
   }
 
   /// Emit an error, then close the watcher.
   void _emitError(error, StackTrace stackTrace) {
+    if (MacOSDirectoryWatcher.logDebugInfo) {
+      print("[$_id] emitting error: $error\n" +
+          "${new Chain.forTrace(stackTrace).terse}");
+    }
     _eventsController.addError(error, stackTrace);
     close();
   }
 
   // TODO(nweiz): remove this when issue 15042 is fixed.
   /// Return a human-friendly string representation of [event].
   String _formatEvent(FileSystemEvent event) {
     if (event == null) return 'null';
 
     var path = p.relative(event.path, from: directory);
     var type = event.isDirectory ? 'directory' : 'file';
     if (event is FileSystemCreateEvent) {
       return "create $type $path";
     } else if (event is FileSystemDeleteEvent) {
       return "delete $type $path";
     } else if (event is FileSystemModifyEvent) {
       return "modify $type $path";
     } else if (event is FileSystemMoveEvent) {
       return "move $type $path to "
           "${p.relative(event.destination, from: directory)}";
     }
   }
 }