Make mappedBy lazy.

sdk/lib/html/templates/html/impl/impl_Element.darttemplate

Index: sdk/lib/html/templates/html/impl/impl_Element.darttemplate
diff --git a/sdk/lib/html/templates/html/impl/impl_Element.darttemplate b/sdk/lib/html/templates/html/impl/impl_Element.darttemplate
index 5700dfd5308fd5931a2776f29f1eda401495a122..b055e471b234cc6c4d62dd3f8dfdaf6637ed23af 100644
--- a/sdk/lib/html/templates/html/impl/impl_Element.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_Element.darttemplate
@@ -59,13 +59,7 @@ class _ChildrenElementList implements List {
     return false;
   }
 
-  Collection mappedBy(f(Element element)) {
-    final out = [];
-    for (Element el in this) {
-      out.add(f(el));
-    }
-    return out;
-  }
+  Iterable mappedBy(f(Element element)) => new MappedIterable(this, f);
 
   bool get isEmpty {
     return _element.$dom_firstElementChild == null;
@@ -176,13 +170,7 @@ class _FrozenElementList implements List {
     }
   }
 
-  Collection mappedBy(f(Element element)) {
-    final out = [];
-    for (Element el in this) {
-      out.add(f(el));
-    }
-    return out;
-  }
+  Iterable mappedBy(f(Element element)) => new MappedIterable(this, f);
 
   List<Element> where(bool f(Element element)) {
     final out = [];