Implement support for converting db values to datastore values with db.Query.filter

lib/src/db/model_db.dart

 // Copyright (c) 2014, 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.
 
 part of gcloud.db;
 
 /**
  * A database of all registered models.
  *
  * Responsible for converting between dart model objects and datastore entities.
@@ skipping 22 matching lines @@
   /**
    * Returns the kind name for instances of [type].
    */
   String kindName(Type type);
 
   /**
    * Returns the property name used for [fieldName]
    */
   // TODO: Get rid of this eventually.
   String fieldNameToPropertyName(String kind, String fieldName);
+
+  /**
+   * Converts [value] according to the [Property] named [fieldName] in [type].
+   */
+  Object toDatastoreValue(Type type, String fieldName, Object value);
 }