Fix bug in test expectation for uri_platform test.

tests/standalone/io/uri_platform_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
+import 'package:path/path.dart' as path;
 import "dart:io";
 
 main() {
   if (Platform.isWindows) {
     Expect.equals("a\\b", Uri.parse("a/b").toFilePath());
     Expect.equals("a\\b\\", Uri.parse("a/b/").toFilePath());
     Expect.equals("a b", Uri.parse("a%20b").toFilePath());
     Expect.equals("\\a b", Uri.parse("file:///a%20b").toFilePath());
     Expect.equals("\\a\\b", Uri.parse("file:///a/b").toFilePath());
     Expect.equals("C:\\", Uri.parse("file:///C:").toFilePath());
@@ skipping 13 matching lines @@
     Expect.equals("/a b", Uri.parse("file:///a%20b").toFilePath());
     Expect.equals("/a/b", Uri.parse("file:///a/b").toFilePath());
     Expect.equals("/C:", Uri.parse("file:///C:").toFilePath());
     Expect.equals("/C:/", Uri.parse("file:///C:/").toFilePath());
     Expect.throws(() => Uri.parse("file://host/a/b").toFilePath(),
                   (e) => e is UnsupportedError);
 
     Expect.equals("a/b", new Uri.file("a/b").toFilePath());
     Expect.equals("a\\b", new Uri.file("a\\b").toFilePath());
   }
-
-  Expect.equals(Uri.base,
-                new Uri.file(Directory.current.path + Platform.pathSeparator));
+  // If the current path is only the root prefix (/ (or c:\), then don't add a
+  // separator at the end.
+  Expect.equals(
+      Uri.base,
+      (Directory.current.path.toString() !=
+       path.rootPrefix(Directory.current.path.toString()))
+      ? new Uri.file(Directory.current.path + Platform.pathSeparator)
+      : new Uri.file(Directory.current.path));
 }