fix how DDC finds its undefined constant

pkg/dev_compiler/test/codegen_expected/script.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   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}`);
   };
   dart.fn(script.main, ListOfStringTovoid());
   dart.trackLibraries("script", {
     "script.dart": script
   }, null);
   // Exports:
   return {
     script: script
   };
 });