Yet another path related fix.
Yet another path related fix.

Turns out that `link_reflectable.dart` was happy without a package-root
only because it did not use any `package:..` imports; with the added
package import of `path.dart`, we must specify a package-root. Added
such a specification to test_reflectable/.test_config.

Review URL: https://codereview.chromium.org/1253003002 .

google/reflectable.dart,bwu-dart-contributing/reflectable,jakemac53/reflectable,luisvt/reflectable,jakemac53/reflectable,bwu-dart-contributing/reflectable,luisvt/reflectable,google/reflectable.dart