Add custom annotations to some APIs for native tree shaking.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 678ab6ba98ca0c601b1a5ba355655d185bdf93b2..6bdad865646150b50ac75557d16a6c9abd4c66e4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -8784,10 +8784,8 @@ class DocumentFragment extends Node {
     this.children = value;
   }
 
-  // Native field is used only by Dart code so does not lead to instantiation
-  // of native classes
-  @Creates('Null')
   List<Element> _children;
+
   List<Element> get children {
     if (_children == null) {
       _children = new FilteredElementList(this);