fix #29182, generate top level const fields lazily

pkg/dev_compiler/test/codegen_expected/async_helper.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 async_helper = Object.create(null);
   let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.functionType(dart.dynamic, [])))();
   let StringToException = () => (StringToException = dart.constFn(dart.definiteFunctionType(core.Exception, [core.String])))();
   let _Action0Tovoid = () => (_Action0Tovoid = dart.constFn(dart.definiteFunctionType(dart.void, [async_helper._Action0])))();
   let VoidTovoid = () => (VoidTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [])))();
   let dynamicTovoid = () => (dynamicTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [dart.dynamic])))();
   let FnTovoid = () => (FnTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [VoidTodynamic()])))();
-  async_helper._initialized = false;
+  dart.defineLazy(async_helper, {
+    get _initialized() {
+      return false;
+    },
+    set _initialized(_) {}
+  });
   async_helper._Action0 = dart.typedef('_Action0', () => dart.functionType(dart.void, []));
-  async_helper._onAsyncEnd = null;
-  async_helper._asyncLevel = 0;
+  dart.defineLazy(async_helper, {
+    get _onAsyncEnd() {
+      return null;
+    },
+    set _onAsyncEnd(_) {},
+    get _asyncLevel() {
+      return 0;
+    },
+    set _asyncLevel(_) {}
+  });
   async_helper._buildException = function(msg) {
     return core.Exception.new(dart.str`Fatal: ${msg}. This is most likely a bug in your test.`);
   };
   dart.fn(async_helper._buildException, StringToException());
   async_helper.asyncTestInitialize = function(callback) {
     async_helper._asyncLevel = 0;
     async_helper._initialized = false;
     async_helper._onAsyncEnd = callback;
   };
   dart.fn(async_helper.asyncTestInitialize, _Action0Tovoid());
@@ skipping 41 matching lines @@
     async_helper.asyncStart();
     dart.dsend(f(), 'then', async_helper.asyncSuccess);
   };
   dart.fn(async_helper.asyncTest, FnTovoid());
   dart.trackLibraries("async_helper", {"async_helper.dart": async_helper}, null);
   // Exports:
   return {
     async_helper: async_helper
   };
 });