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

lib/src/db/model_db.dart

Index: lib/src/db/model_db.dart
diff --git a/lib/src/db/model_db.dart b/lib/src/db/model_db.dart
index 0655ac3ad8c8faf3c0da4b18f02a7c6064e95d35..8408ac530de1990d234e6a93d169253dbc618f33 100644
--- a/lib/src/db/model_db.dart
+++ b/lib/src/db/model_db.dart
@@ -40,4 +40,9 @@ abstract class ModelDB {
    */
   // 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);
 }