Generate contains() method for list mixin only if not already defined.

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..d72fc5d1f84179d2eaeb58666f8b292f17a23cef 100644
--- a/lib/html/templates/immutable_list_mixin.darttemplate
+++ b/lib/html/templates/immutable_list_mixin.darttemplate
@@ -24,6 +24,12 @@
     throw const UnsupportedOperationException("Cannot add to immutable List.");
   }
 
+$if DEFINE_CONTAINS
+  bool contains($E element) => _Collections.contains(this, element);
+$else
+  // contains() defined by IDL.
+$endif
+
   void forEach(void f($E element)) => _Collections.forEach(this, f);
 
   Collection map(f($E element)) => _Collections.map(this, [], f);