anchor.diff:        MixinApplication mixin = cls;
         elements.add(js.string(mixin.mixinClass.name));
         elements.add(js.number(mixin.mixinClass.holder.index));
  +      if (cls.isDirectlyInstantiated) {
  +        elements.add(_generateConstructor(cls));
  +      }
       } else {
         elements.add(_generateConstructor(cls));
       }
  
positive.code: 
  import '../js_emitter.dart' show NativeGenerator, buildTearOffCode;
  import '../model.dart';
  
  
  
  class ModelEmitter {
  
negative.code:     return constructor;
    }
  
    function fillPrototypeWithMixedIn(mixinName, mixinHolderIndex, prototype) {
      var mixin = holders[mixinHolderIndex][mixinName].ensureResolved();
      var mixinPrototype = mixin.prototype;
  
  