fix dummy setters -- grammar requires an identifier

lib/src/codegen/js_codegen.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.
 
 library ddc.src.codegen.js_codegen;
 
 import 'dart:io' show Directory, File;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 955 matching lines @@
     var methods = [];
     for (var node in fields) {
       var name = node.name.name;
       methods.add(new JS.Method(new JS.PropertyName(name),
           js.call('function() { return #; }', _visit(node.initializer)),
           isGetter: true));
 
       // TODO(jmesserly): use a dummy setter to indicate writable.
       if (!node.isFinal) {
         methods.add(new JS.Method(
-            new JS.PropertyName(name), js.call('function() {}'),
+            new JS.PropertyName(name), js.call('function(_) {}'),
             isSetter: true));
       }
     }
 
     return js.statement(
         'dart.defineLazyProperties(#, { # })', [objExpr, methods]);
   }
 
   void _flushLibraryProperties(List<JS.Statement> body) {
     if (_properties.isEmpty) return;
@@ skipping 865 matching lines @@
 
     // TODO(jmesserly): in many cases marking the end will be unncessary.
     printer.mark(_location(node.end));
   }
 
   String _getIdentifier(AstNode node) {
     if (node is SimpleIdentifier) return node.name;
     return null;
   }
 }