Throw FileSystemException from FileSystemEntity methods.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index c538715b36d4bf56c3f23e2e41b8cbcdbf175900..a83e6bb3d7aa4b37bc9679a8096a8a29c1d0f7ba 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -6,7 +6,6 @@ library fasta.source_loader;
 
 import 'dart:async' show Future;
 
-import 'dart:io';
 import 'dart:typed_data' show Uint8List;
 
 import 'package:front_end/file_system.dart';
@@ -92,13 +91,7 @@ class SourceLoader<L> extends Loader<L> {
         bytes = zeroTerminatedBytes;
         sourceBytes[uri] = bytes;
       } on FileSystemException catch (e) {
-        // TODO(scheglov) Throw and catch abstract file-system exceptions.
-        String message = e.message;
-        String osMessage = e.osError?.message;
-        if (osMessage != null && osMessage.isNotEmpty) {
-          message = osMessage;
-        }
-        return inputError(uri, -1, message);
+        return inputError(uri, -1, e.message);
       }
     }