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

test/codegen/expect/misc.js

 dart_library.library('misc', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class _Uninitialized extends core.Object {
     _Uninitialized() {
     }
   }
   dart.setSignature(_Uninitialized, {
     constructors: () => ({_Uninitialized: [_Uninitialized, []]})
   });
-  let UNINITIALIZED = dart.const(new _Uninitialized());
-  let Generic$ = dart.generic(function(T) {
+  const UNINITIALIZED = dart.const(new _Uninitialized());
+  const Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       get type() {
         return Generic$();
       }
     }
     return Generic;
   });
   let Generic = Generic$();
   class Base extends core.Object {
     Base() {
       this.x = 1;
       this.y = 2;
     }
     ['=='](obj) {
       return dart.is(obj, Base) && dart.equals(dart.dload(obj, 'x'), this.x) && dart.equals(dart.dload(obj, 'y'), this.y);
     }
   }
   class Derived extends core.Object {
     Derived() {
       this.z = 3;
     }
     ['=='](obj) {
       return dart.is(obj, Derived) && dart.equals(dart.dload(obj, 'z'), this.z) && super['=='](obj);
     }
   }
   function _isWhitespace(ch) {
     return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
   }
   dart.fn(_isWhitespace, core.bool, [core.String]);
-  let expr = 'foo';
-  let _escapeMap = dart.const(dart.map({'\n': '\\n', '\r': '\\r', '\f': '\\f', '\b': '\\b', '\t': '\\t', '\v': '\\v', '': '\\x7F', [`\${${expr}}`]: ''}));
+  const expr = 'foo';
+  const _escapeMap = dart.const(dart.map({'\n': '\\n', '\r': '\\r', '\f': '\\f', '\b': '\\b', '\t': '\\t', '\v': '\\v', '': '\\x7F', [`\${${expr}}`]: ''}));
   function main() {
     core.print(dart.toString(1));
     core.print(dart.toString(1.0));
     core.print(dart.toString(1.1));
     let x = 42;
     core.print(dart.equals(x, dart.dynamic));
     core.print(dart.equals(x, Generic));
     core.print(new (Generic$(core.int))().type);
     core.print(dart.equals(new Derived(), new Derived()));
   }
   dart.fn(main);
   // Exports:
   exports.UNINITIALIZED = UNINITIALIZED;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.Base = Base;
   exports.Derived = Derived;
   exports.expr = expr;
   exports.main = main;
 });