Fix factory constructors unparse:

lib/compiler/implementation/dart_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.
 
 /**
  * Dart backend helper for converting program IR back to source code.
  */
 class Emitter {
 
   final Compiler compiler;
   final StringBuffer sb;
 
   Emitter(this.compiler) : sb = new StringBuffer();
 
   /**
    * Outputs given class element with selected inner elements.
    */
   void outputClass(ClassElement classElement, Set<Element> innerElements) {
     // TODO(smok): Very soon properly print out correct class declaration with
     // extends, implements, etc.
-    sb.add('class ');
+    sb.add(classElement.beginToken.slowToString());  // 'class' or 'interface'.
+    sb.add(' ');
     sb.add(classElement.name.slowToString());
     sb.add('{');
     innerElements.forEach((element) {
       // TODO(smok): Filter out default constructors here.
       outputElement(element);
     });
     sb.add('}');
   }
 
   void outputElement(Element element) {
@@ skipping 17 matching lines @@
       // but unparse initializer manually.
       sb.add(element.parseNode(compiler).unparse());
       sb.add(';');
     } else {
       sb.add(element.parseNode(compiler).unparse());
     }
   }
 
   String toString() => sb.toString();
 }