Revert "Remove deprecated mappedBy."

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

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 605731c4928078663a82f59cfaa12d0c49398dc1..5e5f0fccee53626954f863c86267d531af11f13b 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -65,6 +65,10 @@ class _ChildrenElementList implements List {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(Element element)) {
+    return IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   Iterable<Element> where(bool f(Element element)) {
     return IterableMixinWorkaround.where(this, f);
   }
@@ -286,6 +290,10 @@ class _FrozenElementList implements List {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(Element element)) {
+    return IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   Iterable<Element> where(bool f(Element element)) {
     return IterableMixinWorkaround.where(this, f);
   }