Added proper FileOutput stub, addresses simc/logger#94

Author: Bungeefan

lib/logger.dart

@@ -5,14 +5,12 @@ export 'src/ansi_color.dart';
 export 'src/filters/development_filter.dart';
 export 'src/filters/production_filter.dart';
 export 'src/log_filter.dart';
-// [LogOutput] class needs to be exposed unconditionally
 export 'src/log_output.dart';
-// ignore: duplicate_export
-export 'src/log_output.dart'
-    if (dart.library.io) 'src/outputs/file_output.dart';
 export 'src/log_printer.dart';
 export 'src/logger.dart';
 export 'src/outputs/console_output.dart';
+export 'src/outputs/file_output_stub.dart'
+    if (dart.library.io) 'src/outputs/file_output.dart';
 export 'src/outputs/memory_output.dart';
 export 'src/outputs/multi_output.dart';
 export 'src/outputs/stream_output.dart';

lib/src/outputs/file_output_stub.dart

@@ -0,0 +1,20 @@
+import 'dart:convert';
+import 'dart:io';
+
+import '../log_output.dart';
+import '../logger.dart';
+
+class FileOutput extends LogOutput {
+  FileOutput({
+    required File file,
+    bool overrideExisting = false,
+    Encoding encoding = utf8,
+  }) {
+    throw UnsupportedError("Not supported on this platform.");
+  }
+
+  @override
+  void output(OutputEvent event) {
+    throw UnsupportedError("Not supported on this platform.");
+  }
+}