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

test/codegen/expect/DeltaBlue.js

 dart_library.library('DeltaBlue', null, /* Imports */[
   "dart/_runtime",
   'BenchmarkBase',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, BenchmarkBase, core) {
   'use strict';
   let dartx = dart.dartx;
   function main() {
     new DeltaBlue().report();
@@ skipping 36 matching lines @@
     constructors: () => ({Strength: [Strength, [core.int, core.String]]}),
     methods: () => ({nextWeaker: [Strength, []]}),
     statics: () => ({
       stronger: [core.bool, [Strength, Strength]],
       weaker: [core.bool, [Strength, Strength]],
       weakest: [Strength, [Strength, Strength]],
       strongest: [Strength, [Strength, Strength]]
     }),
     names: ['stronger', 'weaker', 'weakest', 'strongest']
   });
-  let REQUIRED = dart.const(new Strength(0, "required"));
-  let STRONG_PREFERRED = dart.const(new Strength(1, "strongPreferred"));
-  let PREFERRED = dart.const(new Strength(2, "preferred"));
-  let STRONG_DEFAULT = dart.const(new Strength(3, "strongDefault"));
-  let NORMAL = dart.const(new Strength(4, "normal"));
-  let WEAK_DEFAULT = dart.const(new Strength(5, "weakDefault"));
-  let WEAKEST = dart.const(new Strength(6, "weakest"));
+  const REQUIRED = dart.const(new Strength(0, "required"));
+  const STRONG_PREFERRED = dart.const(new Strength(1, "strongPreferred"));
+  const PREFERRED = dart.const(new Strength(2, "preferred"));
+  const STRONG_DEFAULT = dart.const(new Strength(3, "strongDefault"));
+  const NORMAL = dart.const(new Strength(4, "normal"));
+  const WEAK_DEFAULT = dart.const(new Strength(5, "weakDefault"));
+  const WEAKEST = dart.const(new Strength(6, "weakest"));
   class Constraint extends core.Object {
     Constraint(strength) {
       this.strength = strength;
     }
     addConstraint() {
       this.addToGraph();
       exports.planner.incrementalAdd(this);
     }
     satisfy(mark) {
       this.chooseMethod(dart.as(mark, core.int));
@@ skipping 101 matching lines @@
     }
     isInput() {
       return true;
     }
     execute() {}
   }
   dart.setSignature(EditConstraint, {
     constructors: () => ({EditConstraint: [EditConstraint, [Variable, Strength]]}),
     methods: () => ({execute: [dart.void, []]})
   });
-  let NONE = 1;
-  let FORWARD = 2;
-  let BACKWARD = 0;
+  const NONE = 1;
+  const FORWARD = 2;
+  const BACKWARD = 0;
   class BinaryConstraint extends Constraint {
     BinaryConstraint(v1, v2, strength) {
       this.v1 = v1;
       this.v2 = v2;
       this.direction = NONE;
       super.Constraint(strength);
       this.addConstraint();
     }
     chooseMethod(mark) {
       if (this.v1.mark == mark) {
@@ skipping 361 matching lines @@
   exports.BinaryConstraint = BinaryConstraint;
   exports.ScaleConstraint = ScaleConstraint;
   exports.EqualityConstraint = EqualityConstraint;
   exports.Variable = Variable;
   exports.Planner = Planner;
   exports.Plan = Plan;
   exports.chainTest = chainTest;
   exports.projectionTest = projectionTest;
   exports.change = change;
 });