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

test/codegen/expect/sunflower/dom.js

 dart_library.library('sunflower/dom', window, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class Overload extends core.Object {
     Overload() {
     }
   }
   dart.setSignature(Overload, {
     constructors: () => ({Overload: [Overload, []]})
   });
-  let overload = dart.const(new Overload());
+  const overload = dart.const(new Overload());
   class CustomEvent extends core.Object {}
   dart.setSignature(CustomEvent, {
     constructors: () => ({CustomEvent: [CustomEvent, [core.String], {detail: dart.dynamic, bubbles: dart.dynamic, cancelable: dart.dynamic}]})
   });
   class HTMLCollection extends core.Object {}
   dart.setSignature(HTMLCollection, {
     methods: () => ({get: [Element, [core.num]]})
   });
-  let EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));
-  let InputElement = HTMLInputElement;
-  let CanvasElement = HTMLCanvasElement;
-  let DivElement = HTMLDivElement;
-  let ScriptElement = HTMLScriptElement;
+  const EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));
+  const InputElement = HTMLInputElement;
+  const CanvasElement = HTMLCanvasElement;
+  const DivElement = HTMLDivElement;
+  const ScriptElement = HTMLScriptElement;
   class RenderingContext extends core.Object {}
   class CanvasDrawingStyles extends core.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
       this.lineCap = null;
       this.lineJoin = null;
       this.miterLimit = null;
       this.lineDashOffset = null;
       this.font = null;
       this.textAlign = null;
       this.textBaseline = null;
     }
   }
   class CanvasPathMethods extends core.Object {}
   // Exports:
   exports.Overload = Overload;
   exports.overload = overload;
   exports.CustomEvent = CustomEvent;
   exports.HTMLCollection = HTMLCollection;
   exports.EventListener = EventListener;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
   exports.DivElement = DivElement;
   exports.ScriptElement = ScriptElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
 });