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

test/codegen/expect/collection/algorithms.js

 dart_library.library('collection/algorithms', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/math'
 ], /* Lazy imports */[
 ], function(exports, dart, core, math) {
   'use strict';
   let dartx = dart.dartx;
   function _comparableBinarySearch(list, key) {
     let min = 0;
@@ skipping 94 matching lines @@
           max = mid;
         } else {
           min = dart.notNull(mid) + 1;
         }
       }
       list[dartx.setRange](dart.notNull(min) + 1, dart.notNull(pos) + 1, list, min);
       list[dartx.set](min, element);
     }
   }
   dart.fn(_insertionSort, dart.void, [core.List, dart.functionType(core.int, [dart.dynamic, dart.dynamic]), core.int, core.int, core.int]);
-  let _MERGE_SORT_LIMIT = 32;
+  const _MERGE_SORT_LIMIT = 32;
   function mergeSort(list, opts) {
     let start = opts && 'start' in opts ? opts.start : 0;
     let end = opts && 'end' in opts ? opts.end : null;
     let compare = opts && 'compare' in opts ? opts.compare : null;
     if (end == null)
       end = list[dartx.length];
     if (compare == null)
       compare = core.Comparable.compare;
     let length = dart.notNull(end) - dart.notNull(start);
     if (dart.notNull(length) < 2)
@@ skipping 109 matching lines @@
     target[dartx.setRange](targetOffset, dart.notNull(targetOffset) + (dart.notNull(secondEnd) - dart.notNull(cursor2)), secondList, cursor2);
   }
   dart.fn(_merge, dart.void, [dart.functionType(core.int, [dart.dynamic, dart.dynamic]), core.List, core.int, core.int, core.List, core.int, core.int, core.List, core.int]);
   // Exports:
   exports.binarySearch = binarySearch;
   exports.shuffle = shuffle;
   exports.reverse = reverse;
   exports.insertionSort = insertionSort;
   exports.mergeSort = mergeSort;
 });