Make int and double nullable by default

lib/config.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.
 
 /// Configuration of DDC rule set.
 library ddc.config;
 
 // Options shared by the compiler and runtime.
 class TypeOptions {
-  static const NONNULLABLE_TYPES = const <String>['int', 'double'];
+  // A list of non-nullable type names (e.g., 'int')
+  static const List<String> NONNULLABLE_TYPES = const <String>[];
   final List<String> nonnullableTypes;
 
   TypeOptions({this.nonnullableTypes: NONNULLABLE_TYPES});
 }
 
 List<String> optionsToList(String option,
     {List<String> defaultValue: const <String>[]}) {
   if (option == null) {
     return defaultValue;
   } else if (option.isEmpty) {
     return <String>[];
   } else {
     return option.split(',');
   }
 }