Moved deleting directory to whenComplete

pkg/docgen/test/single_library_test.dart

 library single_library_test;
 
 import 'dart:io'; 
 
 import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import '../lib/docgen.dart';
 
 main() {
@@ skipping 95 matching lines @@
           
           // Testing trying to refer to doThis function
           var methodDocComment = fixReference('doThis', libraryMirror, 
               classMirror, methodMirror).children.first.text;
           expect(methodDocComment == 'test.A.doThis', isTrue);
           
           // Testing something with no reference
           var libraryDocComment = fixReference('foobar', libraryMirror,
               classMirror, methodMirror).children.first.text;
           expect(libraryDocComment == 'foobar', isTrue);
-          
-          temporaryDir.deleteSync(recursive: true);
-        }));
+        })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }