Add "contains" method to Collection.

lib/html/templates/immutable_list_mixin.darttemplate

Index: lib/html/templates/immutable_list_mixin.darttemplate
diff --git a/lib/html/templates/immutable_list_mixin.darttemplate b/lib/html/templates/immutable_list_mixin.darttemplate
index ee45407aa0d2946acb947f411e6b12bcbbe6b224..1e6ebb5c112f3e0634fb4c23263b9b7896d72b95 100644
--- a/lib/html/templates/immutable_list_mixin.darttemplate
+++ b/lib/html/templates/immutable_list_mixin.darttemplate
@@ -24,6 +24,8 @@
     throw const UnsupportedOperationException("Cannot add to immutable List.");
   }
 
+  bool contains($E element) => _Collections.contains(this, element);
+
   void forEach(void f($E element)) => _Collections.forEach(this, f);
 
   Collection map(f($E element)) => _Collections.map(this, [], f);