Do not generate empty native classes. Fixes issue 6872.

sdk/lib/_internal/compiler/implementation/js_backend/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;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 219 matching lines @@
     //    constructor.
     // For engines where we have access to the '__proto__' we can manipulate
     // the object literal directly. For other engines we have to create a new
     // object and copy over the members.
     return '''
 function(collectedClasses) {
   var hasOwnProperty = Object.prototype.hasOwnProperty;
   for (var cls in collectedClasses) {
     if (hasOwnProperty.call(collectedClasses, cls)) {
       var desc = collectedClasses[cls];
-      $isolatePropertiesName[cls] = $defineClassName(cls, desc[''], desc);
+'''/* If the class does not have any declared fields (in the ''
+      property of the description), then provide an empty list of fields. */'''
+      $isolatePropertiesName[cls] = $defineClassName(cls, desc[''] || [], desc);
       if (desc['super'] !== "") $pendingClassesName[cls] = desc['super'];
     }
   }
   var pendingClasses = $pendingClassesName;
 '''/* FinishClasses can be called multiple times. This means that we need to
       clear the pendingClasses property. */'''
   $pendingClassesName = {};
   var finishedClasses = {};
   function finishClass(cls) {
 '''/* Opera does not support 'getOwnPropertyNames'. Therefore we use
@@ skipping 420 matching lines @@
     emitExtraAccessors(member, defineInstanceMember);
   }
 
   /**
    * Documentation wanted -- johnniwinther
    *
    * Invariant: [classElement] must be a declaration element.
    */
   void emitInstanceMembers(ClassElement classElement,
                            CodeBuffer buffer,
-                           bool needsLeadingComma) {
+                           bool emitLeadingComma) {
     assert(invariant(classElement, classElement.isDeclaration));
-    bool needsComma = needsLeadingComma;
     void defineInstanceMember(String name, StringBuffer memberBuffer) {
-      if (needsComma) buffer.add(',');
-      needsComma = true;
+      if (emitLeadingComma) buffer.add(',');
+      emitLeadingComma = true;
       buffer.add('\n');
       buffer.add(' $name: ');
       buffer.add(memberBuffer);
     }
 
     JavaScriptBackend backend = compiler.backend;
     if (classElement == backend.objectInterceptorClass) {
       emitInterceptorMethods(defineInstanceMember);
       // The ObjectInterceptor does not have any instance methods.
       return;
@@ skipping 149 matching lines @@
     String setterName =
         namer.publicSetterName(new SourceString(accessorName));
     buffer.add("$setterName: function(v) { "
         "this.$fieldName = $helperName(v$additionalArgument); }");
   }
 
   void emitClassConstructor(ClassElement classElement, CodeBuffer buffer) {
     /* Do nothing. */
   }
 
-  void emitClassFields(ClassElement classElement, CodeBuffer buffer) {
-    buffer.add('"": [');
+  void emitClassFields(ClassElement classElement,
+                       CodeBuffer buffer,
+                       bool emitEndingComma) {
     bool isFirstField = true;
     visitClassFields(classElement, (Element member,
                                     String name,
                                     String accessorName,
                                     bool needsGetter,
                                     bool needsSetter,
                                     bool needsCheckedSetter) {
       if (isFirstField) {
+        buffer.add('"": [');
         isFirstField = false;
       } else {
         buffer.add(", ");
       }
       buffer.add('"$accessorName');
       int flag = 0;
       if (name != accessorName) {
         buffer.add(':$name');
         assert(needsGetter || needsSetter);
         flag = RENAMING_FLAG;
       }
       if (needsGetter && needsSetter) {
         buffer.addCharCode(GETTER_SETTER_CODE + flag);
       } else if (needsGetter) {
         buffer.addCharCode(GETTER_CODE + flag);
       } else if (needsSetter) {
         buffer.addCharCode(SETTER_CODE + flag);
       }
       buffer.add('"');
     });
-    buffer.add(']');
+    if (!isFirstField) {
+      // There was at least one field.
+      buffer.add(']');
+      if (emitEndingComma) {
+        buffer.add(', ');
+      }
+    }
   }
 
   /** Each getter/setter must be prefixed with a ",\n ". */
-  void emitClassGettersSetters(ClassElement classElement, CodeBuffer buffer,
-                               {bool omitLeadingComma: false}) {
+  void emitClassGettersSetters(ClassElement classElement,
+                               CodeBuffer buffer,
+                               bool emitLeadingComma) {
     visitClassFields(classElement, (Element member,
                                     String name,
                                     String accessorName,
                                     bool needsGetter,
                                     bool needsSetter,
                                     bool needsCheckedSetter) {
       if (needsCheckedSetter) {
         assert(!needsSetter);
-        if (!omitLeadingComma) {
+        if (emitLeadingComma) {
           buffer.add(",\n ");
         } else {
-          omitLeadingComma = false;
+          emitLeadingComma = true;
         }
         generateCheckedSetter(member, name, accessorName, buffer);
       }
     });
   }
 
   /**
    * Documentation wanted -- johnniwinther
    *
    * Invariant: [classElement] must be a declaration element.
@@ skipping 13 matching lines @@
     needsDefineClass = true;
     String className = namer.getName(classElement);
     ClassElement superclass = classElement.superclass;
     String superName = "";
     if (superclass != null) {
       superName = namer.getName(superclass);
     }
 
     buffer.add('$classesCollector.$className = {');
     emitClassConstructor(classElement, buffer);
-    emitClassFields(classElement, buffer);
+    emitClassFields(classElement, buffer, true);
     // TODO(floitsch): the emitInstanceMember should simply always emit a ',\n'.
     // That does currently not work because the native classes have a different
     // syntax.
-    buffer.add(',\n "super": "$superName"');
-    emitClassGettersSetters(classElement, buffer);
+    buffer.add('\n "super": "$superName"');
+    emitClassGettersSetters(classElement, buffer, true);
     emitInstanceMembers(classElement, buffer, true);
     buffer.add('\n};\n\n');
   }
 
   void emitInterceptorMethods(
       void defineInstanceMember(String name, StringBuffer memberBuffer)) {
     JavaScriptBackend backend = compiler.backend;
     // Emit forwarders for the ObjectInterceptor class. We need to
     // emit all possible sends on intercepted methods.
     for (Selector selector in backend.usedInterceptors) {
@@ skipping 793 matching lines @@
 const String HOOKS_API_USAGE = """
 // Generated by dart2js, the Dart to JavaScript compiler.
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;