Recommit changing List.skip/take/revert returns Iterable and remove mappedBy.

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 2c8bf6814002a5c80e31a1382cc1670dcb55f2ca..850c2f82ab3a785cfedf13e1b84796ab0f3a5c85 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -32,9 +32,6 @@ $endif
   Iterable map(f($E element)) =>
       IterableMixinWorkaround.mapList(this, f);
 
-  List mappedBy(f($E element)) =>
-      IterableMixinWorkaround.mappedByList(this, f);
-
   Iterable<$E> where(bool f($E element)) =>
       IterableMixinWorkaround.where(this, f);
 
@@ -107,7 +104,7 @@ $else
   // clear() defined by IDL.
 $endif
 
-  List<$E> get reversed {
+  Iterable<$E> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }