All strong mode cleaning of dart2js.

pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.kernel.backend_strategy;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../backend_strategy.dart';
 import '../closure.dart';
@@ skipping 28 matching lines @@
 /// Backend strategy that uses the kernel elements as the backend model.
 // TODO(johnniwinther): Replace this with a strategy based on the J-element
 // model.
 class KernelBackendStrategy implements BackendStrategy {
   final Compiler _compiler;
   Sorter _sorter;
 
   KernelBackendStrategy(this._compiler);
 
   @override
-  ClosedWorldRefiner createClosedWorldRefiner(KernelClosedWorld closedWorld) {
+  ClosedWorldRefiner createClosedWorldRefiner(
+      covariant KernelClosedWorld closedWorld) {
     return closedWorld;
   }
 
   @override
   Sorter get sorter {
     if (_sorter == null) {
       KernelFrontEndStrategy frontendStrategy = _compiler.frontendStrategy;
       _sorter = new KernelSorter(frontendStrategy.elementMap);
     }
     return _sorter;
@@ skipping 343 matching lines @@
   Iterable<ClassEntity> sortClasses(Iterable<ClassEntity> classes) {
     return classes.toList()
       ..sort((ClassEntity a, ClassEntity b) {
         int r = _compareLibraries(a.library, b.library);
         if (r != 0) return r;
         return _compareNodes(
             a, elementMap.getClassNode(a), b, elementMap.getClassNode(b));
       });
   }
 }