Revert all html warning changes.

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

 // Copyright (c) 2011, 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 $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME() => document.createDocumentFragment();
 
   factory $CLASSNAME.html(String html,
       {NodeValidator validator, NodeTreeSanitizer treeSanitizer}) {
 
     return document.body.createFragment(html,
       validator: validator, treeSanitizer: treeSanitizer);
   }
 
   factory $CLASSNAME.svg(String svgContent,
       {NodeValidator validator, NodeTreeSanitizer treeSanitizer}) {
 
     return new svg.SvgSvgElement().createFragment(svgContent,
         validator: validator, treeSanitizer: treeSanitizer);
   }
 
-  HtmlCollection get _children => throw new UnimplementedError(
-      'Use _docChildren instead');
-
 $if DART2JS
   // Native field is used only by Dart code so does not lead to instantiation
   // of native classes
   @Creates('Null')
 $endif
-  List<Element> _docChildren;
+  List<Element> _children;
 
   List<Element> get children {
-    if (_docChildren == null) {
-      _docChildren = new FilteredElementList(this);
+    if (_children == null) {
+      _children = new FilteredElementList(this);
     }
-    return _docChildren;
+    return _children;
   }
 
   void set children(List<Element> value) {
     // Copy list first since we don't want liveness during iteration.
     List copy = new List.from(value);
     var children = this.children;
     children.clear();
     children.addAll(copy);
   }
 
@@ skipping 64 matching lines @@
    * semantics will be changing in the future.
    */
   @deprecated
   @Experimental()
   @DomName('DocumentFragment.querySelectorAll')
   ElementList queryAll(String relativeSelectors) {
     return querySelectorAll(relativeSelectors);
   }
 $!MEMBERS
 }