Do not generate empty native classes. Fixes issue 6872.

sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.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 NativeEmitter {
 
   CodeEmitterTask emitter;
   CodeBuffer nativeBuffer;
@@ skipping 148 matching lines @@
     String quotedName = classElement.nativeName.slowToString();
     if (isNativeLiteral(quotedName)) {
       generateNativeLiteral(classElement);
       // The native literal kind needs to be dealt with specially when
       // generating code for it.
       return;
     }
 
     CodeBuffer fieldBuffer = new CodeBuffer();
     CodeBuffer getterSetterBuffer = new CodeBuffer();
+    CodeBuffer methodBuffer = new CodeBuffer();
 
-    emitter.emitClassFields(classElement, fieldBuffer);
-    emitter.emitClassGettersSetters(classElement, getterSetterBuffer,
-                                    omitLeadingComma: true);
-
-    CodeBuffer methodBuffer = new CodeBuffer();
-    emitter.emitInstanceMembers(classElement, methodBuffer, false);
+    emitter.emitClassFields(
+        classElement, fieldBuffer, emitEndingComma: false);
+    emitter.emitClassGettersSetters(
+        classElement, getterSetterBuffer, emitLeadingComma: false);
+    emitter.emitInstanceMembers(
+        classElement, methodBuffer, emitLeadingComma: false);
 
     if (methodBuffer.isEmpty
         && fieldBuffer.isEmpty
         && getterSetterBuffer.isEmpty) {
       return;
     }
 
     String nativeName = toNativeName(classElement);
     nativeBuffer.add("$defineNativeClassName('$nativeName', ");
     nativeBuffer.add('{');
@@ skipping 313 matching lines @@
         if (!first) targetBuffer.add(",\n");
         targetBuffer.add(" $name: $function");
         first = false;
       });
       targetBuffer.add("\n});\n\n");
     }
     targetBuffer.add(nativeBuffer);
     targetBuffer.add('\n');
   }
 }