Make mappedBy lazy.

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

Index: sdk/lib/html/templates/html/impl/impl_NodeList.darttemplate
diff --git a/sdk/lib/html/templates/html/impl/impl_NodeList.darttemplate b/sdk/lib/html/templates/html/impl/impl_NodeList.darttemplate
index 94d5c3278a309e517ecf8b38a5edbdd8b6639ffe..186b1b541d52998cfe6c6568f0a1cc7b7f0605cd 100644
--- a/sdk/lib/html/templates/html/impl/impl_NodeList.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_NodeList.darttemplate
@@ -17,7 +17,7 @@ class _ListWrapper<E> implements List<E> {
 
   void forEach(void f(E element)) => _list.forEach(f);
 
-  Collection mappedBy(f(E element)) => _list.mappedBy(f);
+  Iterable mappedBy(f(E element)) => new MappedIterable(this, f);
 
   List<E> where(bool f(E element)) => _list.where(f);
 
@@ -134,7 +134,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   void forEach(void f(Node element)) => _Collections.forEach(this, f);
 
-  Collection mappedBy(f(Node element)) => _Collections.mappedBy(this, [], f);
+  Iterable mappedBy(f(Node element)) => new MappedIterable(this, f);
 
   Collection<Node> where(bool f(Node element)) =>
      new _NodeListWrapper(_Collections.where(this, <Node>[], f));