Moved deleting directory to whenComplete

pkg/docgen/test/single_library_test.dart

Index: pkg/docgen/test/single_library_test.dart
diff --git a/pkg/docgen/test/single_library_test.dart b/pkg/docgen/test/single_library_test.dart
index 8aeefdbcc26acc6e9ddc92e0ceecd8ff9ceae8bf..1462c63f63c7df8041f2c1c74328f071f4c16fd1 100644
--- a/pkg/docgen/test/single_library_test.dart
+++ b/pkg/docgen/test/single_library_test.dart
@@ -113,9 +113,7 @@ main() {
           var libraryDocComment = fixReference('foobar', libraryMirror,
               classMirror, methodMirror).children.first.text;
           expect(libraryDocComment == 'foobar', isTrue);
-          
-          temporaryDir.deleteSync(recursive: true);
-        }));
+        })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }