Add the test for updating a part.

pkg/front_end/lib/src/incremental/file_state.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.
 
 import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/fasta/parser/top_level_parser.dart';
 import 'package:front_end/src/fasta/scanner.dart';
@@ skipping 105 matching lines @@
     //   1) The absolute file URI.
     //   2) The absolute non-file URI, e.g. `package:foo/foo.dart`.
     Uri absoluteUri;
     try {
       absoluteUri = fileUri.resolve(relativeUri);
     } on FormatException {
       return;
     }
 
     // Resolve the absolute URI into the absolute file URI.
-    Uri resolvedUri = _fsState.uriTranslator.translate(absoluteUri);
-    if (resolvedUri == null) return;
+    Uri resolvedUri;
+    if (absoluteUri.isScheme('file')) {
+      resolvedUri = absoluteUri;
+    } else {
+      resolvedUri = _fsState.uriTranslator.translate(absoluteUri);
+      if (resolvedUri == null) return;
+    }
 
     FileState file = await _fsState.getFile(resolvedUri);
     files.add(file);
   }
 
   /// Scan the content of the file.
   ScannerResult _scan() {
     var zeroTerminatedBytes = new Uint8List(_content.length + 1);
     zeroTerminatedBytes.setRange(0, _content.length, _content);
     return scan(zeroTerminatedBytes);
@@ skipping 76 matching lines @@
   Future<String> readAsString() async => _shouldNotBeQueried();
 
   /// _FileSystemViewEntry is used by the incremental kernel generator to
   /// provide Fasta with a consistent, race condition free view of the files
   /// constituting the project.  It should only need to be used for reading
   /// file contents.
   dynamic _shouldNotBeQueried() {
     throw new StateError('The method should not be invoked.');
   }
 }