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

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

 dart_library.library('collection/src/canonicalized_map', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'collection/src/utils',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, utils, collection) {
   'use strict';
   let dartx = dart.dartx;
-  let _base = Symbol('_base');
-  let _canonicalize = Symbol('_canonicalize');
-  let _isValidKeyFn = Symbol('_isValidKeyFn');
-  let _isValidKey = Symbol('_isValidKey');
-  let CanonicalizedMap$ = dart.generic(function(C, K, V) {
+  const _base = Symbol('_base');
+  const _canonicalize = Symbol('_canonicalize');
+  const _isValidKeyFn = Symbol('_isValidKeyFn');
+  const _isValidKey = Symbol('_isValidKey');
+  const CanonicalizedMap$ = dart.generic(function(C, K, V) {
     class CanonicalizedMap extends core.Object {
       CanonicalizedMap(canonicalize, opts) {
         let isValidKey = opts && 'isValidKey' in opts ? opts.isValidKey : null;
         this[_base] = core.Map$(C, utils.Pair$(K, V)).new();
         this[_canonicalize] = canonicalize;
         this[_isValidKeyFn] = isValidKey;
       }
       from(other, canonicalize, opts) {
         let isValidKey = opts && 'isValidKey' in opts ? opts.isValidKey : null;
         this[_base] = core.Map$(C, utils.Pair$(K, V)).new();
@@ skipping 85 matching lines @@
         [_isValidKey]: [core.bool, [core.Object]]
       })
     });
     return CanonicalizedMap;
   });
   let CanonicalizedMap = CanonicalizedMap$();
   // Exports:
   exports.CanonicalizedMap$ = CanonicalizedMap$;
   exports.CanonicalizedMap = CanonicalizedMap;
 });