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

test/codegen/expect/fieldtest.js

 dart_library.library('fieldtest', 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 = 42;
     }
   }
-  let B$ = dart.generic(function(T) {
+  const B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
         this.x = null;
         this.y = null;
         this.z = null;
       }
     }
     return B;
   });
   let B = B$();
@@ skipping 37 matching lines @@
       return 1;
     }
   }
   class Derived extends BaseWithGetter {
     Derived() {
       this.foo = 2;
       this.bar = 3;
     }
   }
   dart.virtualField(Derived, 'foo');
-  let Generic$ = dart.generic(function(T) {
+  const Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       foo(t) {
         dart.as(t, T);
         return core.print(dart.notNull(Generic$().bar) + dart.notNull(dart.as(t, core.String)));
       }
     }
     dart.setSignature(Generic, {
       methods: () => ({foo: [dart.dynamic, [T]]})
     });
     return Generic;
@@ skipping 47 matching lines @@
   exports.compute = compute;
   exports.BaseWithGetter = BaseWithGetter;
   exports.Derived = Derived;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.StaticFieldOrder1 = StaticFieldOrder1;
   exports.StaticFieldOrder2 = StaticFieldOrder2;
   exports.MyEnum = MyEnum;
   exports.main = main;
 });