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

test/codegen/expect/sunflower/painter.js

 dart_library.library('sunflower/painter', null, /* Imports */[
   "dart/_runtime",
   'dart/math',
   'sunflower/dom',
   'dart/core',
   'sunflower/circle'
 ], /* Lazy imports */[
 ], function(exports, dart, math, dom, core, circle) {
   'use strict';
   let dartx = dart.dartx;
-  let ORANGE = "orange";
-  let RED = "red";
-  let BLUE = "blue";
-  let TAU = dart.notNull(math.PI) * 2;
+  const ORANGE = "orange";
+  const RED = "red";
+  const BLUE = "blue";
+  const TAU = dart.notNull(math.PI) * 2;
   function querySelector(selector) {
     return dom.document.querySelector(selector);
   }
   dart.fn(querySelector, dom.Element, [core.String]);
   dart.defineLazyProperties(exports, {
     get canvas() {
       return dart.as(querySelector("#canvas"), dom.CanvasElement);
     },
     get context() {
       return dart.as(exports.canvas.getContext('2d'), dom.CanvasRenderingContext2D);
@@ skipping 19 matching lines @@
     methods: () => ({draw: [dart.void, [dom.CanvasRenderingContext2D]]})
   });
   // Exports:
   exports.ORANGE = ORANGE;
   exports.RED = RED;
   exports.BLUE = BLUE;
   exports.TAU = TAU;
   exports.querySelector = querySelector;
   exports.CirclePainter = CirclePainter;
 });