Fix MemoryCachingByteStore._evict after an unsuccessful get.

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 45 matching lines @@
   final _map = new LinkedHashMap<String, List<int>>();
   int _currentSizeBytes = 0;
 
   MemoryCachingByteStore(this._store, this._maxSizeBytes);
 
   @override
   List<int> get(String key) {
     List<int> bytes = _map.remove(key);
     if (bytes == null) {
       bytes = _store.get(key);
-      _map[key] = bytes;
-      _currentSizeBytes += bytes?.length ?? 0;
-      _evict();
+      if (bytes != null) {
+        _map[key] = bytes;
+        _currentSizeBytes += bytes.length;
+        _evict();
+      }
     } else {
       _map[key] = bytes;
     }
     return bytes;
   }
 
   @override
   void put(String key, List<int> bytes) {
     _store.put(key, bytes);
     _currentSizeBytes -= _map[key]?.length ?? 0;
     _map[key] = bytes;
     _currentSizeBytes += bytes.length;
     _evict();
   }
 
   void _evict() {
     while (_currentSizeBytes > _maxSizeBytes) {
       if (_map.isEmpty) {
+        // Should be impossible, since _currentSizeBytes should always match
+        // _map.  But recover anyway.
+        assert(false);
+        _currentSizeBytes = 0;
         break;
       }
       String key = _map.keys.first;
       List<int> bytes = _map.remove(key);
       _currentSizeBytes -= bytes.length;
     }
   }
 }