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

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

 dart_library.library('collection/src/unmodifiable_wrappers', null, /* Imports */[
   "dart/_runtime",
   'dart/collection',
   'dart/core'
 ], /* Lazy imports */[
   'collection/wrappers'
 ], function(exports, dart, collection, core, wrappers) {
   'use strict';
   let dartx = dart.dartx;
   dart.export(exports, collection, ['UnmodifiableListView', 'UnmodifiableMapView'], []);
-  let NonGrowableListMixin$ = dart.generic(function(E) {
+  const NonGrowableListMixin$ = dart.generic(function(E) {
     class NonGrowableListMixin extends core.Object {
       static _throw() {
         dart.throw(new core.UnsupportedError("Cannot change the length of a fixed-length list"));
       }
       set length(newLength) {
         return NonGrowableListMixin$()._throw();
       }
       add(value) {
         dart.as(value, E);
         return dart.as(NonGrowableListMixin$()._throw(), core.bool);
@@ skipping 68 matching lines @@
       'removeWhere',
       'retainWhere',
       'removeRange',
       'replaceRange',
       'clear',
       'length'
     ]);
     return NonGrowableListMixin;
   });
   let NonGrowableListMixin = NonGrowableListMixin$();
-  let NonGrowableListView$ = dart.generic(function(E) {
+  const NonGrowableListView$ = dart.generic(function(E) {
     class NonGrowableListView extends dart.mixin(wrappers.DelegatingList$(E), NonGrowableListMixin$(E)) {
       NonGrowableListView(listBase) {
         super.DelegatingList(listBase);
       }
     }
     dart.setSignature(NonGrowableListView, {
       constructors: () => ({NonGrowableListView: [exports.NonGrowableListView$(E), [core.List$(E)]]})
     });
     return NonGrowableListView;
   });
   dart.defineLazyClassGeneric(exports, 'NonGrowableListView', {get: NonGrowableListView$});
-  let _throw = Symbol('_throw');
-  let UnmodifiableSetMixin$ = dart.generic(function(E) {
+  const _throw = Symbol('_throw');
+  const UnmodifiableSetMixin$ = dart.generic(function(E) {
     class UnmodifiableSetMixin extends core.Object {
       [_throw]() {
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable Set"));
       }
       add(value) {
         dart.as(value, E);
         return dart.as(this[_throw](), core.bool);
       }
       addAll(elements) {
         dart.as(elements, core.Iterable$(E));
@@ skipping 30 matching lines @@
         removeAll: [dart.void, [core.Iterable]],
         retainAll: [dart.void, [core.Iterable]],
         removeWhere: [dart.void, [dart.functionType(core.bool, [E])]],
         retainWhere: [dart.void, [dart.functionType(core.bool, [E])]],
         clear: [dart.void, []]
       })
     });
     return UnmodifiableSetMixin;
   });
   let UnmodifiableSetMixin = UnmodifiableSetMixin$();
-  let UnmodifiableSetView$ = dart.generic(function(E) {
+  const UnmodifiableSetView$ = dart.generic(function(E) {
     class UnmodifiableSetView extends dart.mixin(wrappers.DelegatingSet$(E), UnmodifiableSetMixin$(E)) {
       UnmodifiableSetView(setBase) {
         super.DelegatingSet(setBase);
       }
     }
     dart.setSignature(UnmodifiableSetView, {
       constructors: () => ({UnmodifiableSetView: [exports.UnmodifiableSetView$(E), [core.Set$(E)]]})
     });
     return UnmodifiableSetView;
   });
   dart.defineLazyClassGeneric(exports, 'UnmodifiableSetView', {get: UnmodifiableSetView$});
-  let UnmodifiableMapMixin$ = dart.generic(function(K, V) {
+  const UnmodifiableMapMixin$ = dart.generic(function(K, V) {
     class UnmodifiableMapMixin extends core.Object {
       static _throw() {
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable Map"));
       }
       set(key, value) {
         (() => {
           dart.as(key, K);
           dart.as(value, V);
           return UnmodifiableMapMixin$()._throw();
         })();
@@ skipping 33 matching lines @@
   // Exports:
   exports.NonGrowableListMixin$ = NonGrowableListMixin$;
   exports.NonGrowableListMixin = NonGrowableListMixin;
   exports.NonGrowableListView$ = NonGrowableListView$;
   exports.UnmodifiableSetMixin$ = UnmodifiableSetMixin$;
   exports.UnmodifiableSetMixin = UnmodifiableSetMixin;
   exports.UnmodifiableSetView$ = UnmodifiableSetView$;
   exports.UnmodifiableMapMixin$ = UnmodifiableMapMixin$;
   exports.UnmodifiableMapMixin = UnmodifiableMapMixin;
 });