Update allocator.dart

Remove unused import.

/tbr @yjbanov.

Author: matanlurey

lib/src/allocator.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:path/path.dart' as p;
-
 import 'specs/directive.dart';
 import 'specs/reference.dart';