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

test/codegen/expect/unittest.js

 dart_library.library('unittest', null, /* Imports */[
   "dart/_runtime",
   'matcher/matcher',
   'dom/dom',
   'dart/core',
   'dart/async',
   'matcher/src/interfaces',
   'matcher/src/util',
   'matcher/src/description'
 ], /* Lazy imports */[
@@ skipping 28 matching lines @@
     TestFailure(message) {
       this.message = message;
     }
     toString() {
       return this.message;
     }
   }
   dart.setSignature(TestFailure, {
     constructors: () => ({TestFailure: [TestFailure, [core.String]]})
   });
-  let ErrorFormatter = dart.typedef('ErrorFormatter', () => dart.functionType(core.String, [dart.dynamic, interfaces.Matcher, core.String, core.Map, core.bool]));
+  const ErrorFormatter = dart.typedef('ErrorFormatter', () => dart.functionType(core.String, [dart.dynamic, interfaces.Matcher, core.String, core.Map, core.bool]));
   function expect(actual, matcher, opts) {
     let reason = opts && 'reason' in opts ? opts.reason : null;
     let verbose = opts && 'verbose' in opts ? opts.verbose : false;
     let formatter = opts && 'formatter' in opts ? opts.formatter : null;
     matcher = util.wrapMatcher(matcher);
     let matchState = dart.map();
     try {
       if (dart.notNull(dart.as(dart.dsend(matcher, 'matches', actual, matchState), core.bool)))
         return;
     } catch (e) {
@@ skipping 27 matching lines @@
   }
   dart.fn(_defaultFailFormatter, core.String, [dart.dynamic, interfaces.Matcher, core.String, core.Map, core.bool]);
   // Exports:
   exports.group = group;
   exports.test = test;
   exports.TestFailure = TestFailure;
   exports.ErrorFormatter = ErrorFormatter;
   exports.expect = expect;
   exports.fail = fail;
 });