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

test/codegen/expect/expect.js

 dart_library.library('expect', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class Expect extends core.Object {
     static _truncateString(string, start, end, length) {
       if (dart.notNull(end) - dart.notNull(start) > dart.notNull(length)) {
@@ skipping 279 matching lines @@
       throws: [dart.void, [dart.functionType(dart.void, [])], [_CheckExceptionFn, core.String]],
       _getMessage: [core.String, [core.String]],
       _fail: [dart.void, [core.String]]
     }),
     names: ['_truncateString', '_stringDifference', 'equals', 'isTrue', 'isFalse', 'isNull', 'isNotNull', 'identical', 'fail', 'approxEquals', 'notEquals', 'listEquals', 'mapEquals', 'stringEquals', 'setEquals', 'throws', '_getMessage', '_fail']
   });
   function _identical(a, b) {
     return core.identical(a, b);
   }
   dart.fn(_identical, core.bool, [dart.dynamic, dart.dynamic]);
-  let _CheckExceptionFn = dart.typedef('_CheckExceptionFn', () => dart.functionType(core.bool, [dart.dynamic]));
-  let _Nullary = dart.typedef('_Nullary', () => dart.functionType(dart.dynamic, []));
+  const _CheckExceptionFn = dart.typedef('_CheckExceptionFn', () => dart.functionType(core.bool, [dart.dynamic]));
+  const _Nullary = dart.typedef('_Nullary', () => dart.functionType(dart.dynamic, []));
   class ExpectException extends core.Object {
     ExpectException(message) {
       this.message = message;
     }
     toString() {
       return this.message;
     }
   }
   ExpectException[dart.implements] = () => [core.Exception];
   dart.setSignature(ExpectException, {
@@ skipping 20 matching lines @@
   dart.setSignature(AssumeDynamic, {
     constructors: () => ({AssumeDynamic: [AssumeDynamic, []]})
   });
   // Exports:
   exports.Expect = Expect;
   exports.ExpectException = ExpectException;
   exports.NoInline = NoInline;
   exports.TrustTypeAnnotations = TrustTypeAnnotations;
   exports.AssumeDynamic = AssumeDynamic;
 });