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

test/codegen/expect/temps.js

 dart_library.library('temps', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
-  let _x = Symbol('_x');
-  let __x = Symbol('__x');
-  let _function = Symbol('_function');
+  const _x = Symbol('_x');
+  const __x = Symbol('__x');
+  const _function = Symbol('_function');
   class FormalCollision extends core.Object {
     FormalCollision(x, _x$, func) {
       this[_x] = x;
       this[__x] = _x$;
       this[_function] = func;
     }
   }
   dart.setSignature(FormalCollision, {
     constructors: () => ({FormalCollision: [FormalCollision, [core.int, core.int, core.Function]]})
   });
-  let _opt = Symbol('_opt');
+  const _opt = Symbol('_opt');
   class OptionalArg extends core.Object {
     OptionalArg(opt) {
       if (opt === void 0)
         opt = 123;
       this[_opt] = opt;
       this.opt = null;
     }
     named(opts) {
       let opt = opts && 'opt' in opts ? opts.opt : 456;
       this.opt = opt;
@@ skipping 11 matching lines @@
     core.print(new FormalCollision(1, 2, dart.fn(x => x)));
     core.print(new OptionalArg()[_opt]);
     core.print(new OptionalArg.named()[_opt]);
   }
   dart.fn(main);
   // Exports:
   exports.FormalCollision = FormalCollision;
   exports.OptionalArg = OptionalArg;
   exports.main = main;
 });