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

test/codegen/expect/collection/wrappers.js

 dart_library.library('collection/wrappers', null, /* Imports */[
   "dart/_runtime",
   'collection/src/canonicalized_map',
   'dart/core',
   'dart/math',
   'dart/collection'
 ], /* Lazy imports */[
   'collection/src/unmodifiable_wrappers'
 ], function(exports, dart, canonicalized_map, core, math, collection, unmodifiable_wrappers) {
   'use strict';
   let dartx = dart.dartx;
   dart.export(exports, canonicalized_map);
   dart.export(exports, unmodifiable_wrappers);
-  let _base = Symbol('_base');
-  let _DelegatingIterableBase$ = dart.generic(function(E) {
+  const _base = Symbol('_base');
+  const _DelegatingIterableBase$ = dart.generic(function(E) {
     class _DelegatingIterableBase extends core.Object {
       _DelegatingIterableBase() {
       }
       any(test) {
         dart.as(test, dart.functionType(core.bool, [E]));
         return this[_base][dartx.any](test);
       }
       contains(element) {
         return this[_base][dartx.contains](element);
       }
@@ skipping 150 matching lines @@
       'isEmpty',
       'isNotEmpty',
       'iterator',
       'last',
       'length',
       'single'
     ]);
     return _DelegatingIterableBase;
   });
   let _DelegatingIterableBase = _DelegatingIterableBase$();
-  let DelegatingIterable$ = dart.generic(function(E) {
+  const DelegatingIterable$ = dart.generic(function(E) {
     class DelegatingIterable extends _DelegatingIterableBase$(E) {
       DelegatingIterable(base) {
         this[_base] = base;
         super._DelegatingIterableBase();
       }
     }
     dart.setSignature(DelegatingIterable, {
       constructors: () => ({DelegatingIterable: [DelegatingIterable$(E), [core.Iterable$(E)]]})
     });
     return DelegatingIterable;
   });
   let DelegatingIterable = DelegatingIterable$();
-  let _listBase = Symbol('_listBase');
-  let DelegatingList$ = dart.generic(function(E) {
+  const _listBase = Symbol('_listBase');
+  const DelegatingList$ = dart.generic(function(E) {
     class DelegatingList extends DelegatingIterable$(E) {
       DelegatingList(base) {
         super.DelegatingIterable(base);
       }
       get [_listBase]() {
         return dart.as(this[_base], core.List$(E));
       }
       get(index) {
         return this[_listBase][dartx.get](index);
       }
@@ skipping 156 matching lines @@
       'setRange',
       'shuffle',
       'sort',
       'sublist',
       'length',
       'reversed'
     ]);
     return DelegatingList;
   });
   let DelegatingList = DelegatingList$();
-  let _setBase = Symbol('_setBase');
-  let DelegatingSet$ = dart.generic(function(E) {
+  const _setBase = Symbol('_setBase');
+  const DelegatingSet$ = dart.generic(function(E) {
     class DelegatingSet extends DelegatingIterable$(E) {
       DelegatingSet(base) {
         super.DelegatingIterable(base);
       }
       get [_setBase]() {
         return dart.as(this[_base], core.Set$(E));
       }
       add(value) {
         dart.as(value, E);
         return this[_setBase].add(value);
@@ skipping 60 matching lines @@
         retainAll: [dart.void, [core.Iterable$(core.Object)]],
         retainWhere: [dart.void, [dart.functionType(core.bool, [E])]],
         union: [core.Set$(E), [core.Set$(E)]],
         toSet: [core.Set$(E), []]
       })
     });
     dart.defineExtensionMembers(DelegatingSet, ['toSet']);
     return DelegatingSet;
   });
   let DelegatingSet = DelegatingSet$();
-  let _baseQueue = Symbol('_baseQueue');
-  let DelegatingQueue$ = dart.generic(function(E) {
+  const _baseQueue = Symbol('_baseQueue');
+  const DelegatingQueue$ = dart.generic(function(E) {
     class DelegatingQueue extends DelegatingIterable$(E) {
       DelegatingQueue(queue) {
         super.DelegatingIterable(queue);
       }
       get [_baseQueue]() {
         return dart.as(this[_base], collection.Queue$(E));
       }
       add(value) {
         dart.as(value, E);
         this[_baseQueue].add(value);
@@ skipping 43 matching lines @@
         remove: [core.bool, [core.Object]],
         removeWhere: [dart.void, [dart.functionType(core.bool, [E])]],
         retainWhere: [dart.void, [dart.functionType(core.bool, [E])]],
         removeFirst: [E, []],
         removeLast: [E, []]
       })
     });
     return DelegatingQueue;
   });
   let DelegatingQueue = DelegatingQueue$();
-  let DelegatingMap$ = dart.generic(function(K, V) {
+  const DelegatingMap$ = dart.generic(function(K, V) {
     class DelegatingMap extends core.Object {
       DelegatingMap(base) {
         this[_base] = base;
       }
       get(key) {
         return this[_base].get(key);
       }
       set(key, value) {
         dart.as(key, K);
         dart.as(value, V);
@@ skipping 55 matching lines @@
         containsKey: [core.bool, [core.Object]],
         containsValue: [core.bool, [core.Object]],
         forEach: [dart.void, [dart.functionType(dart.void, [K, V])]],
         putIfAbsent: [V, [K, dart.functionType(V, [])]],
         remove: [V, [core.Object]]
       })
     });
     return DelegatingMap;
   });
   let DelegatingMap = DelegatingMap$();
-  let _baseMap = Symbol('_baseMap');
-  let MapKeySet$ = dart.generic(function(E) {
+  const _baseMap = Symbol('_baseMap');
+  const MapKeySet$ = dart.generic(function(E) {
     class MapKeySet extends dart.mixin(_DelegatingIterableBase$(E), unmodifiable_wrappers.UnmodifiableSetMixin$(E)) {
       MapKeySet(base) {
         this[_baseMap] = base;
         super._DelegatingIterableBase();
       }
       get [_base]() {
         return this[_baseMap].keys;
       }
       contains(element) {
         return this[_baseMap].containsKey(element);
@@ skipping 46 matching lines @@
     dart.defineExtensionMembers(MapKeySet, [
       'contains',
       'toString',
       'isEmpty',
       'isNotEmpty',
       'length'
     ]);
     return MapKeySet;
   });
   dart.defineLazyClassGeneric(exports, 'MapKeySet', {get: MapKeySet$});
-  let _keyForValue = Symbol('_keyForValue');
-  let MapValueSet$ = dart.generic(function(K, V) {
+  const _keyForValue = Symbol('_keyForValue');
+  const MapValueSet$ = dart.generic(function(K, V) {
     class MapValueSet extends _DelegatingIterableBase$(V) {
       MapValueSet(base, keyForValue) {
         this[_baseMap] = base;
         this[_keyForValue] = keyForValue;
         super._DelegatingIterableBase();
       }
       get [_base]() {
         return this[_baseMap].values;
       }
       contains(element) {
@@ skipping 116 matching lines @@
     dart.defineExtensionMembers(MapValueSet, [
       'contains',
       'toString',
       'isEmpty',
       'isNotEmpty',
       'length'
     ]);
     return MapValueSet;
   });
   let MapValueSet = MapValueSet$();
-  let __CastType0$ = dart.generic(function(V) {
-    let __CastType0 = dart.typedef('__CastType0', () => dart.functionType(V, []));
+  const __CastType0$ = dart.generic(function(V) {
+    const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(V, []));
     return __CastType0;
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.DelegatingIterable$ = DelegatingIterable$;
   exports.DelegatingIterable = DelegatingIterable;
   exports.DelegatingList$ = DelegatingList$;
   exports.DelegatingList = DelegatingList;
   exports.DelegatingSet$ = DelegatingSet$;
   exports.DelegatingSet = DelegatingSet;
   exports.DelegatingQueue$ = DelegatingQueue$;
   exports.DelegatingQueue = DelegatingQueue;
   exports.DelegatingMap$ = DelegatingMap$;
   exports.DelegatingMap = DelegatingMap;
   exports.MapKeySet$ = MapKeySet$;
   exports.MapValueSet$ = MapValueSet$;
   exports.MapValueSet = MapValueSet;
 });