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

test/codegen/expect/covariance.js

Index: test/codegen/expect/covariance.js
diff --git a/test/codegen/expect/covariance.js b/test/codegen/expect/covariance.js
index 76c23aa68f847da275fdcf882bef07a2a4105f9c..59aaf212ff3010f1941b9e5570b14a696af73797 100644
--- a/test/codegen/expect/covariance.js
+++ b/test/codegen/expect/covariance.js
@@ -5,8 +5,8 @@ dart_library.library('covariance', null, /* Imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
-  let _t = Symbol('_t');
-  let Foo$ = dart.generic(function(T) {
+  const _t = Symbol('_t');
+  const Foo$ = dart.generic(function(T) {
     class Foo extends core.Object {
       Foo() {
         this[_t] = null;