Missing file from CL 2804843003.

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

+// Copyright (c) 2017, 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.io;
+
+import 'dart:async' show Future;
+
+import 'dart:io' show FileSystemException;
+
+import 'errors.dart' show inputError;
+
+import 'scanner/io.dart' as scanner_io show readBytesFromFile;
+
+Future<List<int>> readBytesFromFile(Uri uri) async {
+  try {
+    return await scanner_io.readBytesFromFile(uri);
+  } on FileSystemException catch (e) {
+    String message = e.message;
+    String osMessage = e.osError?.message;
+    if (osMessage != null && osMessage.isNotEmpty) {
+      message = osMessage;
+    }
+    return inputError(uri, -1, message);
+  }
+}