fix #29182, generate top level const fields lazily

pkg/dev_compiler/test/codegen_expected/destructuring.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 destructuring = Object.create(null);
   const src__varargs = Object.create(null);
   let intAnddynamic__Todynamic = () => (intAnddynamic__Todynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [core.int, dart.dynamic], [dart.dynamic])))();
   let intAnddynamic__Todynamic$ = () => (intAnddynamic__Todynamic$ = dart.constFn(dart.definiteFunctionType(dart.dynamic, [core.int, dart.dynamic], {c: dart.dynamic})))();
   let intAnddynamicTodynamic = () => (intAnddynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [core.int, dart.dynamic])))();
@@ skipping 57 matching lines @@
   };
   dart.fn(destructuring.invalid_names3, __Todynamic$());
   destructuring.names_clashing_with_object_props = function({constructor = null, valueOf = null, hasOwnProperty = 2} = Object.create(null)) {
     destructuring.f(constructor, valueOf, hasOwnProperty);
   };
   dart.fn(destructuring.names_clashing_with_object_props, __Todynamic$0());
   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("destructuring", {"destructuring.dart": destructuring, "package:js/src/varargs.dart": src__varargs}, null);
   // Exports:
   return {
     destructuring: destructuring,
     src__varargs: src__varargs
   };
 });