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

test/codegen/expect/collection/priority_queue.js

 dart_library.library('collection/priority_queue', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection) {
   'use strict';
   let dartx = dart.dartx;
-  let PriorityQueue$ = dart.generic(function(E) {
+  const PriorityQueue$ = dart.generic(function(E) {
     class PriorityQueue extends core.Object {}
     return PriorityQueue;
   });
   let PriorityQueue = PriorityQueue$();
-  let _queue = Symbol('_queue');
-  let _length = Symbol('_length');
-  let _add = Symbol('_add');
-  let _locate = Symbol('_locate');
-  let _removeLast = Symbol('_removeLast');
-  let _bubbleUp = Symbol('_bubbleUp');
-  let _bubbleDown = Symbol('_bubbleDown');
-  let _grow = Symbol('_grow');
-  let HeapPriorityQueue$ = dart.generic(function(E) {
+  const _queue = Symbol('_queue');
+  const _length = Symbol('_length');
+  const _add = Symbol('_add');
+  const _locate = Symbol('_locate');
+  const _removeLast = Symbol('_removeLast');
+  const _bubbleUp = Symbol('_bubbleUp');
+  const _bubbleDown = Symbol('_bubbleDown');
+  const _grow = Symbol('_grow');
+  const HeapPriorityQueue$ = dart.generic(function(E) {
     class HeapPriorityQueue extends core.Object {
       HeapPriorityQueue(comparison) {
         if (comparison === void 0)
           comparison = null;
         this[_queue] = core.List$(E).new(HeapPriorityQueue$()._INITIAL_CAPACITY);
         this.comparison = comparison != null ? comparison : core.Comparable.compare;
         this[_length] = 0;
       }
       add(element) {
         dart.as(element, E);
@@ skipping 197 matching lines @@
         [_removeLast]: [E, []],
         [_bubbleUp]: [dart.void, [E, core.int]],
         [_bubbleDown]: [dart.void, [E, core.int]],
         [_grow]: [dart.void, []]
       })
     });
     return HeapPriorityQueue;
   });
   let HeapPriorityQueue = HeapPriorityQueue$();
   HeapPriorityQueue._INITIAL_CAPACITY = 7;
-  let __CastType0$ = dart.generic(function(E) {
-    let __CastType0 = dart.typedef('__CastType0', () => dart.functionType(core.int, [E, E]));
+  const __CastType0$ = dart.generic(function(E) {
+    const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(core.int, [E, E]));
     return __CastType0;
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.PriorityQueue$ = PriorityQueue$;
   exports.PriorityQueue = PriorityQueue;
   exports.HeapPriorityQueue$ = HeapPriorityQueue$;
   exports.HeapPriorityQueue = HeapPriorityQueue;
 });