Fix all strong-mode warnings.

lib/src/utils/file_pool.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 barback.utils.file_pool;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:pool/pool.dart';
-import 'package:stack_trace/stack_trace.dart';
 
 import '../utils.dart';
 
 /// Manages a pool of files that are opened for reading to cope with maximum
 /// file descriptor limits.
 ///
 /// If a file cannot be opened because too many files are already open, this
 /// will defer the open until a previously opened file is closed and then try
 /// again. If this doesn't succeed after a certain amount of time, the open
 /// will fail and the original "too many files" exception will be thrown.
 class FilePool {
   /// The underlying pool.
   ///
   /// The maximum number of allocated descriptors is based on empirical tests
   /// that indicate that beyond 32, additional file reads don't provide
   /// substantial additional throughput.
   final Pool _pool = new Pool(32, timeout: new Duration(seconds: 60));
 
   /// Opens the file at [path] for reading.
   ///
   /// When the returned stream is listened to, if there are too many files
   /// open, this will wait for a previously opened file to be closed and then
   /// try again.
   Stream<List<int>> openRead(String path) {
     return futureStream(_pool.request().then((resource) {
-      return Chain.track(new File(path).openRead()).transform(
+      return new File(path).openRead().transform(
           new StreamTransformer.fromHandlers(handleDone: (sink) {
         sink.close();
         resource.release();
       }));
     }));
   }
 
   /// Reads [path] as a string using [encoding].
   ///
   /// If there are too many files open and the read fails, this will wait for
   /// a previously opened file to be closed and then try again.
   Future<String> readAsString(String path, Encoding encoding) {
     return _readAsBytes(path).then(encoding.decode);
   }
 
   /// Reads [path] as a list of bytes, using [openRead] to retry if there are
   /// failures.
   Future<List<int>> _readAsBytes(String path) {
     var completer = new Completer<List<int>>();
     var builder = new BytesBuilder();
 
     openRead(path).listen(builder.add, onDone: () {
       completer.complete(builder.takeBytes());
     }, onError: completer.completeError, cancelOnError: true);
 
     return completer.future;
   }
 }