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 6b75c1324416915a1c09353e648e2bc68bada7ea..7e3d10d5f9f0f6619f635a79b2d3c501ae0433e4 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> {