Remove meta dependency

Author: simc

lib/logger.dart

@@ -4,7 +4,6 @@ library logger;
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
-import 'package:meta/meta.dart';
 
 import 'src/ansi_color.dart';
 

lib/src/outputs/file_output.dart

@@ -8,7 +8,7 @@ class FileOutput extends LogOutput {
   IOSink _sink;
 
   FileOutput({
-    @required this.file,
+    this.file,
     this.overrideExisting = false,
     this.encoding = utf8,
   });