fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/script.js

Index: pkg/dev_compiler/test/codegen_expected/script.js
diff --git a/pkg/dev_compiler/test/codegen_expected/script.js b/pkg/dev_compiler/test/codegen_expected/script.js
index 0d1116c007d86f22d11ce64364f49b1d5789ac75..f04e90c3bc2f91315aeb581d3f009b4783837488 100644
--- a/pkg/dev_compiler/test/codegen_expected/script.js
+++ b/pkg/dev_compiler/test/codegen_expected/script.js
@@ -4,10 +4,11 @@ define(['dart_sdk'], function(dart_sdk) {
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const script = Object.create(null);
+  const $join = dartx.join;
   let ListOfString = () => (ListOfString = dart.constFn(core.List$(core.String)))();
   let ListOfStringTovoid = () => (ListOfStringTovoid = dart.constFn(dart.fnType(dart.void, [ListOfString()])))();
   script.main = function(args) {
-    let name = args[dartx.join](' ');
+    let name = args[$join](' ');
     if (name === '') name = 'world';
     core.print(dart.str`hello ${name}`);
   };