fix dummy setters -- grammar requires an identifier

lib/src/js/nodes.dart

 // Copyright (c) 2012, 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.
 
 part of js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 1183 matching lines @@
   int get precedenceLevel => PRIMARY;
 }
 
 class Method extends Property {
   final bool isGetter;
   final bool isSetter;
   final bool isStatic;
 
   Method(Expression name, Fun function,
       {this.isGetter: false, this.isSetter: false, this.isStatic: false})
-      : super(name, function);
+      : super(name, function) {
+    assert(!isGetter || function.params.length == 0);
+    assert(!isSetter || function.params.length == 1);
+  }
 
   Fun get function => super.value;
 
   accept(NodeVisitor visitor) => visitor.visitMethod(this);
 
   void visitChildren(NodeVisitor visitor) {
     name.accept(visitor);
     function.accept(visitor);
   }
 
@@ skipping 189 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }