make constructors private for better intellisense

Author: 2002Bishwajeet

templates/dart/lib/id.dart.twig

@@ -1,6 +1,8 @@
 part of {{ language.params.packageName }};
 
 class ID {
+    const ID._();
+    
     static String unique() {
         return 'unique()';
     }

templates/dart/lib/permission.dart.twig

@@ -1,6 +1,8 @@
 part of {{ language.params.packageName }};
 
 class Permission {
+    const Permission._();
+
     static String read(String role) {
         return 'read("$role")';
     }

templates/dart/lib/query.dart.twig

@@ -1,6 +1,8 @@
 part of {{ language.params.packageName }};
 
 class Query {
+    const Query._();
+    
   static equal(String attribute, dynamic value) =>
       _addQuery(attribute, 'equal', value);
 

templates/dart/lib/role.dart.twig

@@ -1,6 +1,8 @@
 part of {{ language.params.packageName }};
 
 class Role {
+    const Role._();
+    
     static String any() {
         return 'any';
     }