fixes #216, JSNumber members

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 99ece9c88e0c5082292229f8b6284f56fb1168b7..a4f09d0a54d5bbdf4c8329ddb7e98a48f8358f35 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -569,15 +569,17 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
     var name = classElem.name;
     var body = <JS.Statement>[];
 
-    if (jsPeerName != null) {
+    if (_extensionTypes.contains(classElem)) {
       var dartxNames = [];
       for (var m in methods) {
         if (!m.isAbstract && !m.isStatic && m.element.isPublic) {
           dartxNames.add(_elementMemberName(m.element, allowExtensions: false));
         }
       }
-      body.add(js.statement('dart.defineExtensionNames(#)',
-          [new JS.ArrayInitializer(dartxNames, multiline: true)]));
+      if (dartxNames.isNotEmpty) {
+        body.add(js.statement('dart.defineExtensionNames(#)',
+        [new JS.ArrayInitializer(dartxNames, multiline: true)]));
+      }
     }
 
     body.add(new JS.ClassDeclaration(cls));