Throw FileSystemException from FileSystemEntity methods.

pkg/front_end/lib/src/fasta/translate_uri.dart

 // Copyright (c) 2016, 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 fasta.translate_uri;
 
 import 'dart:async' show Future;
-import 'dart:io' show FileSystemException;
 
 import 'package:front_end/file_system.dart';
 import 'package:package_config/packages_file.dart' as packages_file show parse;
 
 import 'errors.dart' show inputError;
 
 class TranslateUri {
   final Map<String, Uri> packages;
   final Map<String, Uri> dartLibraries;
 
@@ skipping 75 matching lines @@
         "web_audio":
             sdk.resolve("lib/web_audio/dartium/web_audio_dartium.dart"),
         "web_gl": sdk.resolve("lib/web_gl/dartium/web_gl_dartium.dart"),
         "web_sql": sdk.resolve("lib/web_sql/dartium/web_sql_dartium.dart"),
       };
     }
     uri ??= Uri.base.resolve(".packages");
 
     List<int> bytes;
     try {
-      FileSystemException;
       bytes = await fileSystem.entityForUri(uri).readAsBytes();
     } on FileSystemException catch (e) {
-      String message = e.message;
-      String osMessage = e.osError?.message;
-      if (osMessage != null && osMessage.isNotEmpty) {
-        message = osMessage;
-      }
-      inputError(uri, -1, message);
+      inputError(uri, -1, e.message);
     }
 
     Map<String, Uri> packages = const <String, Uri>{};
     try {
       packages = packages_file.parse(bytes, uri);
     } on FormatException catch (e) {
       return inputError(uri, e.offset, e.message);
     }
     return new TranslateUri(packages, dartLibraries);
   }
 }