Throw FileSystemException from FileSystemEntity methods.

pkg/front_end/lib/src/fasta/source/source_loader.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.source_loader;
 
 import 'dart:async' show Future;
 
-import 'dart:io';
 import 'dart:typed_data' show Uint8List;
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/instrumentation.dart' show Instrumentation;
 
 import 'package:front_end/src/fasta/builder/ast_factory.dart' show AstFactory;
 
 import 'package:front_end/src/fasta/kernel/kernel_ast_factory.dart'
     show KernelAstFactory;
 
@@ skipping 65 matching lines @@
     // Get the library text from the cache, or read from the file system.
     List<int> bytes = sourceBytes[uri];
     if (bytes == null) {
       try {
         List<int> rawBytes = await fileSystem.entityForUri(uri).readAsBytes();
         Uint8List zeroTerminatedBytes = new Uint8List(rawBytes.length + 1);
         zeroTerminatedBytes.setRange(0, rawBytes.length, rawBytes);
         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);
       }
     }
 
     byteCount += bytes.length - 1;
     ScannerResult result = scan(bytes);
     Token token = result.tokens;
     if (!suppressLexicalErrors) {
       List<int> source = getSource(bytes);
       target.addSourceInformation(library.fileUri, result.lineStarts, source);
     }
@@ skipping 308 matching lines @@
   /// Performs the second phase of top level initializer inference, which is to
   /// visit fields and top level variables in topologically-sorted order and
   /// assign their types.
   void performInitializerInference() {
     typeInferenceEngine.finishTopLevel();
     ticker.logMs("Performed initializer inference");
   }
 
   List<Uri> getDependencies() => sourceBytes.keys.toList();
 }