Add Iterable.fold (and Stream.fold) which replace `reduce`.

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 02f43e98ddaaef1c5f8f42149d3a57f0d74d1dc3..6d34148c72457c2b0e0d4ecf50f8053071d8dbe3 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -162,6 +162,11 @@ class _ChildrenElementList implements List {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, Element element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   void setRange(int start, int rangeLength, List from, [int startFrom = 0]) {
     throw new UnimplementedError();
   }
@@ -408,6 +413,11 @@ class _FrozenElementList implements List {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, Element element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   void setRange(int start, int rangeLength, List from, [int startFrom = 0]) {
     throw new UnsupportedError('');
   }