Do not generate empty native classes. Fixes issue 6872.

sdk/lib/_internal/compiler/implementation/js_backend/emitter_no_eval.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_backend;
 
 class CodeEmitterNoEvalTask extends CodeEmitterTask {
   CodeEmitterNoEvalTask(Compiler compiler,
                         Namer namer,
                         bool generateSourceMap)
@@ skipping 103 matching lines @@
     // Generate the constructor.
     buffer.add("'': function $constructorName(");
     buffer.add(Strings.join(argumentNames, ", "));
     buffer.add(") {\n");
     for (int i = 0; i < fields.length; i++) {
       buffer.add("  this.${fields[i]} = ${argumentNames[i]};\n");
     }
     buffer.add(' }');
   }
 
-  void emitClassFields(ClassElement classElement, CodeBuffer buffer) {
-    /* Do nothing. */
+  void emitClassFields(ClassElement classElement, CodeBuffer buffer,
+                       {bool emitEndingComma: true}) {
+    if (emitEndingComma) buffer.add(', ');
   }
 
   void emitClassGettersSetters(ClassElement classElement, CodeBuffer buffer,
-                               {bool omitLeadingComma: false}) {
+                               {bool emitLeadingComma: true}) {
     emitComma() {
-      if (!omitLeadingComma) {
+      if (emitLeadingComma) {
         buffer.add(",\n ");
       } else {
-        omitLeadingComma = false;
+        emitLeadingComma = true;
       }
     }
 
     visitClassFields(classElement, (Element member,
                                     String name,
                                     String accessorName,
                                     bool needsGetter,
                                     bool needsSetter,
                                     bool needsCheckedSetter) {
       if (needsGetter) {
         emitComma();
         generateGetter(member, name, accessorName, buffer);
       }
       if (needsSetter) {
         emitComma();
         generateSetter(member, name, accessorName, buffer);
       }
       if (needsCheckedSetter) {
         assert(!needsSetter);
         emitComma();
         generateCheckedSetter(member, name, accessorName, buffer);
       }
     });
   }
 }