Fix all strong-mode warnings.

lib/src/utils/file_pool.dart

Index: lib/src/utils/file_pool.dart
diff --git a/lib/src/utils/file_pool.dart b/lib/src/utils/file_pool.dart
index 6a233379daa786e96ee4ad2deba3c4704516b030..c7f3d3931908a019f31850a7a947660c0465f405 100644
--- a/lib/src/utils/file_pool.dart
+++ b/lib/src/utils/file_pool.dart
@@ -9,7 +9,6 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:pool/pool.dart';
-import 'package:stack_trace/stack_trace.dart';
 
 import '../utils.dart';
 
@@ -35,7 +34,7 @@ class FilePool {
   /// 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();