fix #29182, generate top level const fields lazily

pkg/dev_compiler/test/codegen_expected/es6_modules.js

 export const es6_modules = Object.create(null);
 import { core, dart, dartx } from 'dart_sdk';
 let B = () => (B = dart.constFn(es6_modules.B$()))();
 let _B = () => (_B = dart.constFn(es6_modules._B$()))();
 let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
 let VoidToString = () => (VoidToString = dart.constFn(dart.definiteFunctionType(core.String, [])))();
 es6_modules.Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
 es6_modules.A = class A extends core.Object {};
 es6_modules._A = class _A extends core.Object {};
 es6_modules.B$ = dart.generic(T => {
   class B extends core.Object {}
   dart.addTypeTests(B);
   return B;
 });
 es6_modules.B = B();
 es6_modules._B$ = dart.generic(T => {
   class _B extends core.Object {}
   dart.addTypeTests(_B);
   return _B;
 });
 es6_modules._B = _B();
 es6_modules.f = function() {
 };
 dart.fn(es6_modules.f, VoidTodynamic());
 es6_modules._f = function() {
 };
 dart.fn(es6_modules._f, VoidTodynamic());
-es6_modules.constant = "abc";
-es6_modules.finalConstant = "abc";
 dart.defineLazy(es6_modules, {
+  get constant() {
+    return "abc";
+  },
+  get finalConstant() {
+    return "abc";
+  },
   get lazy() {
     return dart.fn(() => {
       core.print('lazy');
       return "abc";
     }, VoidToString())();
-  }
-});
-es6_modules.mutable = "abc";
-dart.defineLazy(es6_modules, {
+  },
+  get mutable() {
+    return "abc";
+  },
+  set mutable(_) {},
   get lazyMutable() {
     return dart.fn(() => {
       core.print('lazyMutable');
       return "abc";
     }, VoidToString())();
   },
   set lazyMutable(_) {}
 });
 dart.trackLibraries("es6_modules", {"es6_modules.dart": es6_modules}, null);