Fix path.toUri for relative paths.

test/posix_test.dart

Index: test/posix_test.dart
diff --git a/test/posix_test.dart b/test/posix_test.dart
index 25e3419910b2b435830b0d32989922e68b82b03a..2368d9992b465ccc40728242bf1c7d9df74b7834 100644
--- a/test/posix_test.dart
+++ b/test/posix_test.dart
@@ -499,6 +499,7 @@ main() {
   test('toUri', () {
     expect(context.toUri('/path/to/foo'), Uri.parse('file:///path/to/foo'));
     expect(context.toUri('/path/to/foo/'), Uri.parse('file:///path/to/foo/'));
+    expect(context.toUri('path/to/foo/'), Uri.parse('path/to/foo/'));
     expect(context.toUri('/'), Uri.parse('file:///'));
     expect(context.toUri('foo/bar'), Uri.parse('foo/bar'));
     expect(context.toUri('/path/to/foo#bar'),