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

test/codegen/expect/collection/src/queue_list.js

 dart_library.library('collection/src/queue_list', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection) {
   'use strict';
   let dartx = dart.dartx;
-  let _head = Symbol('_head');
-  let _tail = Symbol('_tail');
-  let _table = Symbol('_table');
-  let _add = Symbol('_add');
-  let _preGrow = Symbol('_preGrow');
-  let _grow = Symbol('_grow');
-  let _writeToList = Symbol('_writeToList');
-  let QueueList$ = dart.generic(function(E) {
+  const _head = Symbol('_head');
+  const _tail = Symbol('_tail');
+  const _table = Symbol('_table');
+  const _add = Symbol('_add');
+  const _preGrow = Symbol('_preGrow');
+  const _grow = Symbol('_grow');
+  const _writeToList = Symbol('_writeToList');
+  const QueueList$ = dart.generic(function(E) {
     class QueueList extends dart.mixin(core.Object, collection.ListMixin$(E)) {
       QueueList(initialCapacity) {
         if (initialCapacity === void 0)
           initialCapacity = null;
         this[_head] = 0;
         this[_tail] = 0;
         this[_table] = null;
         if (initialCapacity == null || dart.notNull(initialCapacity) < dart.notNull(QueueList$()._INITIAL_CAPACITY)) {
           initialCapacity = QueueList$()._INITIAL_CAPACITY;
         } else if (!dart.notNull(QueueList$()._isPowerOf2(initialCapacity))) {
@@ skipping 206 matching lines @@
       'length'
     ]);
     return QueueList;
   });
   let QueueList = QueueList$();
   QueueList._INITIAL_CAPACITY = 8;
   // Exports:
   exports.QueueList$ = QueueList$;
   exports.QueueList = QueueList;
 });