Implement getRange (returning an Iterable).

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 7376bc48fadec25daabddba5995c88d746d6050b..5d1b5c5be4be2a3cd04069fd423682a3cbf77b5c 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -183,14 +183,14 @@ $endif
     throw new UnsupportedError("Cannot insertRange on immutable List.");
   }
 
+  Iterable<$E> getRange(int start, int end) =>
+    IterableMixinWorkaround.getRangeList(this, start, end);
+
   List<$E> sublist(int start, [int end]) {
     if (end == null) end = length;
     return Lists.getRange(this, start, end, <$E>[]);
   }
 
-  List<$E> getRange(int start, int rangeLength) =>
-      sublist(start, start + rangeLength);
-
   Map<int, $E> asMap() =>
     IterableMixinWorkaround.asMapList(this);