Reapply removeWhere and retainWhere for elements in dart:html

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 021a3ca53e198019a839085e18344c8c674afc6c..288b826ca44cd240d00d64266afbe7bb963b6e93 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -7251,6 +7251,24 @@ class _ChildrenElementList extends ListBase<Element> {
     throw new UnsupportedError('Cannot sort element lists');
   }
 
+  void removeWhere(bool test(Element element)) {
+    _filter(test, false);
+  }
+
+  void retainWhere(bool test(Element element)) {
+    _filter(test, true);
+  }
+
+  void _filter(bool test(var element), bool retainMatching) {
+    Iterable removed = [];
+    if (retainMatching) {
+      removed = _element.children.where((e) => !test(e));
+    } else {
+      removed = _element.children.where(test);
+    }
+    for (var e in removed) e.remove();
+  }
+
   void setRange(int start, int end, Iterable<Element> iterable,
                 [int skipCount = 0]) {
     throw new UnimplementedError();