[html] Better code for Element / _ChildrenElementList method

Two tricks let us compile this more efficiently:

    e.children.addAll(cs);

Normally, get$children is inlined, returning a _ChildrenElementList
wrapper, generating:

     new W._ChildrenElementList(e, e.children).addAll$1(0, cs);

The two tricks:

1. Split _ChildElementList.addAll into an 'unwrap' that then calls the
   logic in '_addAll'

2. Add information about the properties of e.children that allow it to
   be removed.

With these tricks, dart2js can optimize the code to this version that
avoids allocating a wrapper or accessing the 'children' property:

     W._ChildrenElementList__addAll(e, cs);

Change-Id: Ifdf533ac4f9790f09f87302e67304b5696097266
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153904
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

sdk/lib/html/dart2js/html_dart2js.dart

@@ -11722,6 +11722,10 @@ class _ChildrenElementList extends ListBase<Element>
   Iterator<Element> get iterator => toList().iterator;
 
   void addAll(Iterable<Element> iterable) {
+    _addAll(_element, iterable);
+  }
+
+  static void _addAll(Element _element, Iterable<Element> iterable) {
     if (iterable is _ChildNodeListLazy) {
       iterable = new List.from(iterable);
     }
@@ -11775,6 +11779,10 @@ class _ChildrenElementList extends ListBase<Element>
   }
 
   bool remove(Object? object) {
+    return _remove(_element, object);
+  }
+
+  static bool _remove(Element _element, Object? object) {
     if (object is Element) {
       Element element = object;
       if (identical(element.parentNode, _element)) {
@@ -11823,7 +11831,10 @@ class _ChildrenElementList extends ListBase<Element>
     return result;
   }
 
-  Element get first {
+  Element get first => _first(_element);
+
+  @pragma('dart2js:noInline')
+  static Element _first(Element _element) {
     Element? result = _element._firstElementChild;
     if (result == null) throw new StateError("No elements");
     return result;
@@ -12965,6 +12976,16 @@ class Element extends Node
    */
   List<Element> get children => new _ChildrenElementList._wrap(this);
 
+  List<Node> get _children =>
+      // Element.children always returns the same list-like object which is a
+      // live view on the underlying DOM tree. So we can GVN it and remove it if
+      // unused.
+      JS(
+          'returns:HtmlCollection;creates:HtmlCollection;'
+              'depends:none;effects:none;gvn:true',
+          '#.children',
+          this);
+
   set children(List<Element> value) {
     // Copy list first since we don't want liveness during iteration.
     var copy = value.toList();
@@ -14824,11 +14845,6 @@ class Element extends Node
   @JSName('childElementCount')
   int get _childElementCount native;
 
-  @JSName('children')
-  @Returns('HtmlCollection')
-  @Creates('HtmlCollection')
-  List<Node> get _children native;
-
   @JSName('firstElementChild')
   Element? get _firstElementChild native;
 

tools/dom/scripts/htmlrenamer.py

@@ -387,7 +387,6 @@ private_html_members = monitored.Set(
         # Not prefixed but requires custom implementation for cross-browser compatibility.
         'Document.visibilityState',
         'Element.animate',
-        'Element.children',
         'Element.childElementCount',
         'Element.firstElementChild',
         'Element.getClientRects',
@@ -791,6 +790,7 @@ removed_html_members = monitored.Set(
         'DOMException.WRONG_DOCUMENT_ERR',
         'Element.accessKey',
         'Element.append',
+        'Element.children',
         'Element.dataset',
         'Element.get:classList',
         'Element.getAttributeNode',

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

@@ -46,6 +46,10 @@ class _ChildrenElementList extends ListBase<Element>
   Iterator<Element> get iterator => toList().iterator;
 
   void addAll(Iterable<Element> iterable) {
+    _addAll(_element, iterable);
+  }
+
+  static void _addAll(Element _element, Iterable<Element> iterable) {
     if (iterable is _ChildNodeListLazy) {
       iterable = new List.from(iterable);
     }
@@ -99,6 +103,10 @@ class _ChildrenElementList extends ListBase<Element>
   }
 
   bool remove(Object$NULLABLE object) {
+    return _remove(_element, object);
+  }
+
+  static bool _remove(Element _element, Object$NULLABLE object) {
     if (object is Element) {
       Element element = object;
       if (identical(element.parentNode, _element)) {
@@ -149,7 +157,10 @@ class _ChildrenElementList extends ListBase<Element>
     return result;
   }
 
-  Element get first {
+  Element get first => _first(_element);
+
+  @pragma('dart2js:noInline')
+  static Element _first(Element _element) {
     Element$NULLABLE result = _element._firstElementChild;
     if (result == null) throw new StateError("No elements");
     return result;
@@ -667,6 +678,15 @@ $endif
    */
   List<Element> get children => new _ChildrenElementList._wrap(this);
 
+  List<Node> get _children =>
+      // Element.children always returns the same list-like object which is a
+      // live view on the underlying DOM tree. So we can GVN it and remove it if
+      // unused.
+      JS('returns:HtmlCollection;creates:HtmlCollection;'
+             'depends:none;effects:none;gvn:true',
+         '#.children',
+         this);
+
   set children(List<Element> value) {
     // Copy list first since we don't want liveness during iteration.
     var copy = value.toList();