Compact incremental cache when we're done using it.

pkg/analyzer/test/src/summary/incremental_cache_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/summary/incremental_cache.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 265 matching lines @@
   }
 }
 
 /**
  * A [Map] based [CacheStorage].
  */
 class _TestCacheStorage implements CacheStorage {
   final Map<String, List<int>> map = <String, List<int>>{};
 
   @override
+  void compact() {}
+
+  @override
   List<int> get(String key) {
     return map[key];
   }
 
   @override
   void put(String key, List<int> bytes) {
     map[key] = bytes;
   }
 }