All strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_backend/element_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.js_backend.element_strategy;
 
 import '../backend_strategy.dart';
 import '../closure.dart' show ClosureConversionTask, ClosureTask;
 import '../common.dart';
 import '../common/codegen.dart';
@@ skipping 20 matching lines @@
 import '../world.dart';
 
 /// Strategy for using the [Element] model from the resolver as the backend
 /// model.
 class ElementBackendStrategy implements BackendStrategy {
   final Compiler _compiler;
   SourceInformationStrategy _sourceInformationStrategy;
 
   ElementBackendStrategy(this._compiler);
 
-  ClosedWorldRefiner createClosedWorldRefiner(ClosedWorldImpl closedWorld) =>
+  ClosedWorldRefiner createClosedWorldRefiner(
+          covariant ClosedWorldImpl closedWorld) =>
       closedWorld;
 
   Sorter get sorter => const ElementSorter();
 
   @override
   ClosureConversionTask createClosureConversionTask(Compiler compiler) =>
       new ClosureTask(compiler);
 
   @override
   CodegenWorldBuilder createCodegenWorldBuilder(
@@ skipping 107 matching lines @@
 
   MemberElement get element => resolvedAst.element;
 
   WorldImpact run() {
     registry = new CodegenRegistry(_closedWorld.elementEnvironment, element);
     return _backend.codegen(this, _closedWorld);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }