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

test/codegen/expect/cascade.js

 dart_library.library('cascade', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class A extends core.Object {
     A() {
       this.x = null;
@@ skipping 68 matching lines @@
   dart.fn(test_VariableDeclaration_first, dart.void, []);
   function test_increment() {
     let a = new A();
     let y = ((() => {
       a.x = dart.dsend(a.x, '+', 1);
       a.x = dart.dsend(a.x, '-', 1);
       return a;
     })());
   }
   dart.fn(test_increment, dart.void, []);
-  let Base$ = dart.generic(function(T) {
+  const Base$ = dart.generic(function(T) {
     class Base extends core.Object {
       Base() {
         this.x = dart.list([], T);
       }
     }
     return Base;
   });
   let Base = Base$();
   class Foo extends Base$(core.int) {
     Foo() {
@@ skipping 16 matching lines @@
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 });