Do not generate empty native classes. Fixes issue 6872.

sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart	(revision 15255)
+++ sdk/lib/_internal/compiler/implementation/js_backend/native_emitter.dart	(working copy)
@@ -166,12 +166,10 @@
 
     CodeBuffer fieldBuffer = new CodeBuffer();
     CodeBuffer getterSetterBuffer = new CodeBuffer();
-
-    emitter.emitClassFields(classElement, fieldBuffer);
-    emitter.emitClassGettersSetters(classElement, getterSetterBuffer,
-                                    omitLeadingComma: true);
-
     CodeBuffer methodBuffer = new CodeBuffer();
+
+    emitter.emitClassFields(classElement, fieldBuffer, false);
+    emitter.emitClassGettersSetters(classElement, getterSetterBuffer, false);
     emitter.emitInstanceMembers(classElement, methodBuffer, false);
 
     if (methodBuffer.isEmpty