Revert "Making all DOM types abstract and removing superfluous Dartium constructors."

tools/dom/templates/html/impl/impl_HTMLTemplateElement.darttemplate

 // 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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 @Experimental
-$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+$(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
 
   // For real TemplateElement use the actual DOM .content field instead of
   // our polyfilled expando.
   @Experimental
   DocumentFragment get content => $dom_content;
 
   static StreamController<DocumentFragment> _instanceCreated;
 
   /**
@@ skipping 105 matching lines @@
     document.head.append(style);
   }
 
   /**
    * A mapping of names to Custom Syntax objects. See [CustomBindingSyntax] for
    * more information.
    */
   @Experimental
   static Map<String, CustomBindingSyntax> syntax = {};
 }