Throw FileSystemException from FileSystemEntity methods.

pkg/front_end/lib/physical_file_system.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 front_end.physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'file_system.dart';
@@ skipping 25 matching lines @@
   _PhysicalFileSystemEntity(this.uri);
 
   @override
   int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is _PhysicalFileSystemEntity && other.uri == uri;
 
   @override
-  Future<List<int>> readAsBytes() => new io.File.fromUri(uri).readAsBytes();
-
-  @override
-  Future<String> readAsString() => new io.File.fromUri(uri).readAsString();
-
-  @override
   Future<bool> exists() async {
     if (await io.FileSystemEntity.isFile(uri.toFilePath())) {
       return new io.File.fromUri(uri).exists();
     } else {
       return new io.Directory.fromUri(uri).exists();
     }
   }
 
   @override
-  Future<DateTime> lastModified() => new io.File.fromUri(uri).lastModified();
+  Future<DateTime> lastModified() async {
+    try {
+      return await new io.File.fromUri(uri).lastModified();
+    } on io.FileSystemException catch (exception) {
+      throw _toFileSystemException(exception);
+    }
+  }
+
+  @override
+  Future<List<int>> readAsBytes() async {
+    try {
+      return await new io.File.fromUri(uri).readAsBytes();
+    } on io.FileSystemException catch (exception) {
+      throw _toFileSystemException(exception);
+    }
+  }
+
+  @override
+  Future<String> readAsString() async {
+    try {
+      return await new io.File.fromUri(uri).readAsString();
+    } on io.FileSystemException catch (exception) {
+      throw _toFileSystemException(exception);
+    }
+  }
+
+  /**
+   * Return the [FileSystemException] for the given I/O exception.
+   */
+  FileSystemException _toFileSystemException(io.FileSystemException exception) {
+    String message = exception.message;
+    String osMessage = exception.osError?.message;
+    if (osMessage != null && osMessage.isNotEmpty) {
+      message = osMessage;
+    }
+    return new FileSystemException(uri, message);
+  }
 }