pkg/docgen: skip only_lib_content_in_pkg_test DEBUG - slow

pkg/docgen/test/only_lib_content_in_pkg_test.dart

Index: pkg/docgen/test/only_lib_content_in_pkg_test.dart
diff --git a/pkg/docgen/test/only_lib_content_in_pkg_test.dart b/pkg/docgen/test/only_lib_content_in_pkg_test.dart
index e429f5f47fde2e47e5c367de7a5e0aa5e52bfd12..0e6e05ab15b3472b5394470982f043fbc69d5ce9 100644
--- a/pkg/docgen/test/only_lib_content_in_pkg_test.dart
+++ b/pkg/docgen/test/only_lib_content_in_pkg_test.dart
@@ -27,10 +27,13 @@ void main() {
       expect(p.basename(thisPath), 'only_lib_content_in_pkg_test.dart');
       expect(p.dirname(thisPath), endsWith('test'));
 
+      var packageRoot = Platform.packageRoot;
+      if (packageRoot == '') packageRoot = null;
+
       var codeDir = p.normalize(p.join(thisPath, '..', '..'));
       expect(FileSystemEntity.isDirectorySync(codeDir), isTrue);
       return dg.docgen(['$codeDir/'], out: p.join(d.defaultRoot, 'docs'),
-          packageRoot: Platform.packageRoot);
+          packageRoot: packageRoot);
     });
 
     d.dir('docs', [