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

test/codegen/expect/closure.js

 dart_library.library('closure', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/js'
 ], /* Lazy imports */[
 ], function(exports, dart, core, js) {
   'use strict';
   let dartx = dart.dartx;
   /** @typedef {function({i: (?number|undefined)}=)} */
-  let Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
-  let Foo$ = dart.generic(function(T) {
+  const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
+  const Foo$ = dart.generic(function(T) {
     class Foo extends core.Object {
       /**
        * @param {?number} i
        * @param {?} v
        */
       Foo(i, v) {
         this.i = i;
         this.v = v;
         this.b = null;
         this.s = null;
@@ skipping 104 matching lines @@
     }
   }
   dart.setSignature(Baz, {
     constructors: () => ({Baz: [Baz, [core.int]]})
   });
   /** @param {?} args */
   function main(args) {
   }
   dart.fn(main, dart.void, [dart.dynamic]);
   /** @final {string} */
-  let some_top_level_constant = "abc";
+  const some_top_level_constant = "abc";
   /** @final {string} */
   exports.some_top_level_final = "abc";
   /** @type {string} */
   exports.some_top_level_var = "abc";
   // Exports:
   exports.Callback = Callback;
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.Baz = Baz;
   exports.main = main;
   exports.some_top_level_constant = some_top_level_constant;
 });