Make 'where' 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 27bdd8672ace146d6706e3c41575ab05a765c55e..bfa12e3c8acdb4943fc188b801301d0fb12fb6a8 100644
--- a/sdk/lib/html/templates/html/impl/impl_Element.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_Element.darttemplate
@@ -31,15 +31,8 @@ class _ChildrenElementList implements List {
     }
   }
 
-  List<Element> where(bool f(Element element)) {
-    final output = [];
-    forEach((Element element) {
-      if (f(element)) {
-        output.add(element);
-      }
-    });
-    return new _FrozenElementList._wrap(output);
-  }
+  Iterable<Element> where(bool f(Element element))
+      => new WhereIterable<Element>(this, f);
 
   bool every(bool f(Element element)) {
     for (Element element in this) {
@@ -176,13 +169,8 @@ class _FrozenElementList implements List {
     return new MappedIterable<Element, dynamic>(this, f);
   }
 
-  List<Element> where(bool f(Element element)) {
-    final out = [];
-    for (Element el in this) {
-      if (f(el)) out.add(el);
-    }
-    return out;
-  }
+  Iterable<Element> where(bool f(Element element))
+      => new WhereIterable<Element>(this, f);
 
   bool every(bool f(Element element)) {
     for(Element element in this) {