Use const for const/final top-levels, types, symbols.

test/codegen/expect/async_helper.js

 dart_library.library('async_helper', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   exports._initialized = false;
-  let _Action0 = dart.typedef('_Action0', () => dart.functionType(dart.void, []));
+  const _Action0 = dart.typedef('_Action0', () => dart.functionType(dart.void, []));
   exports._onAsyncEnd = null;
   exports._asyncLevel = 0;
   function _buildException(msg) {
     return core.Exception.new(`Fatal: ${msg}. This is most likely a bug in your test.`);
   }
   dart.fn(_buildException, core.Exception, [core.String]);
   function asyncTestInitialize(callback) {
     exports._asyncLevel = 0;
     exports._initialized = false;
     exports._onAsyncEnd = callback;
@@ skipping 44 matching lines @@
     dart.dsend(f(), 'then', asyncSuccess);
   }
   dart.fn(asyncTest, dart.void, [dart.functionType(dart.dynamic, [])]);
   // Exports:
   exports.asyncTestInitialize = asyncTestInitialize;
   exports.asyncStart = asyncStart;
   exports.asyncEnd = asyncEnd;
   exports.asyncSuccess = asyncSuccess;
   exports.asyncTest = asyncTest;
 });