fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/varargs.js

Index: pkg/dev_compiler/test/codegen_expected/varargs.js
diff --git a/pkg/dev_compiler/test/codegen_expected/varargs.js b/pkg/dev_compiler/test/codegen_expected/varargs.js
index 9a54a3f32f628e34845b04fd03902d4f55ae1dad..cfac7a9a439774d3fccbf0ef9192db9ee90a8ba9 100644
--- a/pkg/dev_compiler/test/codegen_expected/varargs.js
+++ b/pkg/dev_compiler/test/codegen_expected/varargs.js
@@ -20,6 +20,7 @@ define(['dart_sdk'], function(dart_sdk) {
   src__varargs._Rest = class _Rest extends core.Object {};
   (src__varargs._Rest.new = function() {
   }).prototype = src__varargs._Rest.prototype;
+  dart.addTypeTests(src__varargs._Rest);
   dart.defineLazy(src__varargs, {
     get rest() {
       return dart.const(new src__varargs._Rest.new());