Add List.asMap().

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 12b0d0c4042ed4fc67d7b5c2681066e7f90be29e..35e8e2ffc9c4241e892495858b71b43fe1e7887d 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -260,6 +260,10 @@ class _ChildrenElementList implements List {
   Element max([int compare(Element a, Element b)]) {
     return IterableMixinWorkaround.max(this, compare);
   }
+
+  Map<int, Element> asMap() {
+    return IterableMixinWorkaround.asMapList(this);
+  }
 }
 
 // TODO(jacobr): this is an inefficient implementation but it is hard to see
@@ -461,6 +465,10 @@ class _FrozenElementList implements List {
   Element max([int compare(Element a, Element b)]) {
     return IterableMixinWorkaround.max(this, compare);
   }
+
+  Map<int, Element> asMap() {
+    return IterableMixinWorkaround.asMapList(this);
+  }
 }
 
 class _FrozenElementListIterator implements Iterator<Element> {