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 5e5f0fccee53626954f863c86267d531af11f13b..605731c4928078663a82f59cfaa12d0c49398dc1 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -65,10 +65,6 @@ 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);
   }
@@ -290,10 +286,6 @@ 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);
   }