Added expand method to iterables.

tools/dom/templates/immutable_list_mixin.darttemplate

Index: tools/dom/templates/immutable_list_mixin.darttemplate
diff --git a/tools/dom/templates/immutable_list_mixin.darttemplate b/tools/dom/templates/immutable_list_mixin.darttemplate
index 77fdd0dc5f6a97b26b1424514c307a4433a993bf..2c8bf6814002a5c80e31a1382cc1670dcb55f2ca 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -38,6 +38,9 @@ $endif
   Iterable<$E> where(bool f($E element)) =>
       IterableMixinWorkaround.where(this, f);
 
+  Iterable expand(Iterable f($E element)) =>
+      IterableMixinWorkaround.expand(this, f);
+
   bool every(bool f($E element)) => IterableMixinWorkaround.every(this, f);
 
   bool any(bool f($E element)) => IterableMixinWorkaround.any(this, f);