Add NullByteStore, that does not store any data.

pkg/analyzer/lib/src/dart/analysis/byte_store.dart

 // Copyright (c) 2015, 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 'dart:collection';
 
 /**
  * Store of bytes associated with string keys.
  *
  * Each key must be not longer than 100 characters and consist of only `[a-z]`,
@@ skipping 80 matching lines @@
         assert(false);
         _currentSizeBytes = 0;
         break;
       }
       String key = _map.keys.first;
       List<int> bytes = _map.remove(key);
       _currentSizeBytes -= bytes.length;
     }
   }
 }
+
+/**
+ * [ByteStore] which does not store any data.
+ */
+class NullByteStore implements ByteStore {
+  @override
+  List<int> get(String key) => null;
+
+  @override
+  void put(String key, List<int> bytes) {}
+}