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

test/codegen/expect/sunflower/sunflower.js

 dart_library.library('sunflower/sunflower', null, /* Imports */[
   "dart/_runtime",
   'sunflower/dom',
   'dart/core',
   'dart/math',
   'sunflower/painter',
   'sunflower/circle'
 ], /* Lazy imports */[
 ], function(exports, dart, dom, core, math, painter, circle) {
   'use strict';
   let dartx = dart.dartx;
-  let SEED_RADIUS = 2;
-  let SCALE_FACTOR = 4;
-  let MAX_D = 300;
-  let centerX = dart.notNull(MAX_D) / 2;
-  let centerY = centerX;
+  const SEED_RADIUS = 2;
+  const SCALE_FACTOR = 4;
+  const MAX_D = 300;
+  const centerX = dart.notNull(MAX_D) / 2;
+  const centerY = centerX;
   function querySelector(selector) {
     return dom.document.querySelector(selector);
   }
   dart.fn(querySelector, dom.Element, [core.String]);
   exports.seeds = 0;
   dart.defineLazyProperties(exports, {
     get canvas() {
       return dart.as(querySelector("#canvas"), dom.CanvasElement);
     },
     get context() {
@@ skipping 43 matching lines @@
   exports.SEED_RADIUS = SEED_RADIUS;
   exports.SCALE_FACTOR = SCALE_FACTOR;
   exports.MAX_D = MAX_D;
   exports.centerX = centerX;
   exports.centerY = centerY;
   exports.querySelector = querySelector;
   exports.main = main;
   exports.draw = draw;
   exports.SunflowerSeed = SunflowerSeed;
 });