Fix physical_file_system_test.dart on Windows.

pkg/front_end/test/physical_file_system_test.dart

Index: pkg/front_end/test/physical_file_system_test.dart
diff --git a/pkg/front_end/test/physical_file_system_test.dart b/pkg/front_end/test/physical_file_system_test.dart
index c3a2ef195a5819176487aded68f657262bf71288..d1a026bbe8550006ae2d4627923b2ebdcef8f120 100644
--- a/pkg/front_end/test/physical_file_system_test.dart
+++ b/pkg/front_end/test/physical_file_system_test.dart
@@ -127,7 +127,7 @@ class PhysicalFileSystemTest extends _BaseTest {
 
   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() {