Fix unittest path

Change-Id: Ie96aeced2564c15198c4538110495701754ebe9c
Reviewed-on: https://dart-review.googlesource.com/74066
Reviewed-by: Johnni Winther <johnniwinther@google.com>

Author: johnniwinther

tests/compiler/dart2js/model/supermixin_test.dart

@@ -8,7 +8,7 @@ import 'package:compiler/src/common_elements.dart';
 import 'package:compiler/src/elements/entities.dart';
 import 'package:expect/expect.dart';
 import '../helpers/program_lookup.dart';
-import '../memory_compiler.dart';
+import '../helpers/memory_compiler.dart';
 
 const String source = r'''
 import 'package:expect/expect.dart';