fix #29182, generate top level const fields lazily

pkg/dev_compiler/test/codegen_expected/varargs.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 varargs = Object.create(null);
   const src__varargs = Object.create(null);
   let dynamicAnddynamicTodynamic = () => (dynamicAnddynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic, dart.dynamic])))();
   let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic])))();
   varargs.varargsTest = function(x, ...others) {
     let args = [1, others];
     dart.dcall(x, ...args);
   };
   dart.fn(varargs.varargsTest, dynamicAnddynamicTodynamic());
   varargs.varargsTest2 = function(x, ...others) {
     let args = [1, others];
     dart.dcall(x, ...args);
   };
   dart.fn(varargs.varargsTest2, dynamicAnddynamicTodynamic());
   src__varargs._Rest = class _Rest extends core.Object {
     new() {
     }
   };
-  src__varargs.rest = dart.const(new src__varargs._Rest());
+  dart.defineLazy(src__varargs, {
+    get rest() {
+      return dart.const(new src__varargs._Rest());
+    }
+  });
   src__varargs.spread = function(args) {
     dart.throw(new core.StateError('The spread function cannot be called, ' + 'it should be compiled away.'));
   };
   dart.fn(src__varargs.spread, dynamicTodynamic());
   dart.trackLibraries("varargs", {"varargs.dart": varargs, "package:js/src/varargs.dart": src__varargs}, null);
   // Exports:
   return {
     varargs: varargs,
     src__varargs: src__varargs
   };
 });