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

test/codegen/expect/collection/iterable_zip.js

 dart_library.library('collection/iterable_zip', null, /* Imports */[
   "dart/_runtime",
   'dart/collection',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, collection, core) {
   'use strict';
   let dartx = dart.dartx;
-  let _iterables = Symbol('_iterables');
+  const _iterables = Symbol('_iterables');
   class IterableZip extends collection.IterableBase$(core.List) {
     IterableZip(iterables) {
       this[_iterables] = iterables;
       super.IterableBase();
     }
     get iterator() {
       let iterators = this[_iterables][dartx.map](dart.fn(x => dart.dload(x, 'iterator')))[dartx.toList]({growable: false});
       return new _IteratorZip(iterators);
     }
   }
   dart.setSignature(IterableZip, {
     constructors: () => ({IterableZip: [IterableZip, [core.Iterable$(core.Iterable)]]})
   });
   dart.defineExtensionMembers(IterableZip, ['iterator']);
-  let _iterators = Symbol('_iterators');
-  let _current = Symbol('_current');
+  const _iterators = Symbol('_iterators');
+  const _current = Symbol('_current');
   class _IteratorZip extends core.Object {
     _IteratorZip(iterators) {
       this[_iterators] = dart.as(iterators, core.List$(core.Iterator));
       this[_current] = null;
     }
     moveNext() {
       if (dart.notNull(this[_iterators][dartx.isEmpty]))
         return false;
       for (let i = 0; dart.notNull(i) < dart.notNull(this[_iterators][dartx.length]); i = dart.notNull(i) + 1) {
         if (!dart.notNull(this[_iterators][dartx.get](i).moveNext())) {
@@ skipping 12 matching lines @@
     }
   }
   _IteratorZip[dart.implements] = () => [core.Iterator$(core.List)];
   dart.setSignature(_IteratorZip, {
     constructors: () => ({_IteratorZip: [_IteratorZip, [core.List]]}),
     methods: () => ({moveNext: [core.bool, []]})
   });
   // Exports:
   exports.IterableZip = IterableZip;
 });