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

test/codegen/expect/collection/algorithms.js

Index: test/codegen/expect/collection/algorithms.js
diff --git a/test/codegen/expect/collection/algorithms.js b/test/codegen/expect/collection/algorithms.js
index 7996c7a1b42e6cee7ae1c083d44e6d62e3f2967d..dd35473bd345c0179bf4df333fe78afbd3482c5e 100644
--- a/test/codegen/expect/collection/algorithms.js
+++ b/test/codegen/expect/collection/algorithms.js
@@ -112,7 +112,7 @@ dart_library.library('collection/algorithms', null, /* Imports */[
     }
   }
   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;