Add custom annotations to some APIs for native tree shaking.

sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate

Index: sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
diff --git a/sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate b/sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
index 22ec7403bd93f6b1645233718460c84959fc0ab5..2106a73f8b0a9ae25924209257491fe6c976dd2a 100644
--- a/sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_DocumentFragment.darttemplate
@@ -35,10 +35,13 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     this.children = value;
   }
 
+$if DART2JS
   // Native field is used only by Dart code so does not lead to instantiation
   // of native classes
   @Creates('Null')
+$endif
   List<Element> _children;
+
   List<Element> get children {
     if (_children == null) {
       _children = new FilteredElementList(this);