Update ByteStore documentation.

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]`,
- * `[0-9]`, `.` and `_` characters. It cannot have the sequence `..` in it.
+ * `[0-9]`, `.` and `_` characters. The key cannot be an empty string, the
+ * literal `.`, or contain the sequence `..`.
  *
  * Note that associations are not guaranteed to be persistent. The value
  * associated with a key can change or become `null` at any point in time.
+ *
+ * TODO(scheglov) Research using asynchronous API.
  */
 abstract class ByteStore {
   /**
    * Return the bytes associated with the given [key].
    * Return `null` if the association does not exist.
    */
   List<int> get(String key);
 
   /**
    * Associate the given [bytes] with the [key].
@@ skipping 32 matching lines @@
   }
 
   void _evict() {
     if (_keys.length > maxEntries) {
       String key = _keys.first;
       _keys.remove(key);
       _map.remove(key);
     }
   }
 }