Check constness of default args

frog/evaluator.dart

 // Copyright (c) 2011, 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('file_system.dart');
 #import('lang.dart');
 
 interface JsEvaluator {
   var eval(String source);
@@ skipping 41 matching lines @@
     if (_prelude == null) {
       throw new UnsupportedOperationException(
           "Must call Evaluator.initWorld before creating a Evaluator.");
     }
     this._jsEvaluator.eval(_prelude);
     _lib = new Library(new SourceFile("_ifrog_", ""));
     _lib.imports.add(new LibraryImport(world.corelib));
     _lib.resolve();
   }
 
-  void _ensureVariableDefined(Identifier name, [List<Token> modifiers = [],
-      TypeReference type = null]) {
+  void _ensureVariableDefined(Identifier name,
+      [List<Token> modifiers = const [], TypeReference type = null]) {

[jimhug, 2011/12/06 16:23:39]

This is really cool!  This particular code is a notorious source of bugs in
Python programs because no one can remember when the list is initialized.  As a
result, the rule in Python is - never use a list as a default argument value -
even though that is never enforced.  The fact that your change caught this issue
and also let us fix it by just adding const to the list says something excellent
about this feature in the dart language.

     var member = _lib.topType.getMember(name.name);
     if (member is FieldMember || member is PropertyMember) return;
     _removeMember(name.name);
     _lib.topType.addField(
         new VariableDefinition(modifiers, type, [name], [null], name.span));
     _lib.topType.getMember(name.name).resolve();
   }
 
   var eval(String dart) {
     var source = new SourceFile("_ifrog_", dart);
@@ skipping 43 matching lines @@
       world.gen.writeType(type);
       code = world.gen.writer.text;
     } else {
       parsed.visit(methGen);
       code = methGen.writer.text;
     }
 
     return this._jsEvaluator.eval(code);
   }
 }