Fix physical_file_system_test.dart on Windows.

pkg/front_end/test/physical_file_system_test.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.
 // SharedOptions=--supermixin
 
 library front_end.test.physical_file_system_test;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io' as io;
@@ skipping 109 matching lines @@
   test_entityForUri() {
     expect(
         PhysicalFileSystem.instance
             .entityForUri(Uri.parse('${tempUri}file.txt'))
             .uri,
         p.toUri(p.join(tempPath, 'file.txt')));
   }
 
   test_entityForUri_bareUri_absolute() {
     expect(PhysicalFileSystem.instance.entityForUri(Uri.parse('/file.txt')).uri,
-        p.toUri(p.fromUri('/file.txt')));
+        Uri.parse('file:///file.txt'));
   }
 
   test_entityForUri_fileUri_relative() {
     // A weird quirk of the Uri class is that it doesn't seem possible to create
     // a `file:` uri with a relative path, no matter how many slashes you use or
     // if you populate the fields directly.  But just to be certain, try to do
     // so, and make that `file:` uris with relative paths are rejected.
     for (var uri in <Uri>[
       new Uri(scheme: 'file', path: 'file.txt'),
       Uri.parse('file:file.txt'),
@@ skipping 49 matching lines @@
       tempDirectory.deleteSync(recursive: true);
     } on io.FileSystemException {
       // Sometimes on Windows the delete fails with errno 32
       // (ERROR_SHARING_VIOLATION: The process cannot access the file because it
       // is being used by another process).  Wait 1 second and try again.
       await new Future.delayed(new Duration(seconds: 1));
       tempDirectory.deleteSync(recursive: true);
     }
   }
 }