All strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart

 // Copyright (c) 2014, 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.
 
 import '../compile_time_constants.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
@@ skipping 81 matching lines @@
       ConstantExpression constant =
           dartConstantCompiler.compileMetadata(metadata, node, elements);
       jsConstantCompiler.compileMetadata(metadata, node, elements);
       return constant;
     });
   }
 
   // TODO(johnniwinther): Remove this when values are computed from the
   // expressions.
   @override
-  void copyConstantValues(JavaScriptConstantTask task) {
+  void copyConstantValues(covariant JavaScriptConstantTask task) {
     jsConstantCompiler.constantValueMap
         .addAll(task.jsConstantCompiler.constantValueMap);
     dartConstantCompiler.constantValueMap
         .addAll(task.dartConstantCompiler.constantValueMap);
   }
 }
 
 /**
  * The [JavaScriptConstantCompiler] is used to keep track of compile-time
  * constants, initializations of global and static fields, and default values of
@@ skipping 114 matching lines @@
 class ForgetConstantNodeVisitor extends Visitor {
   final JavaScriptConstantCompiler constants;
 
   ForgetConstantNodeVisitor(this.constants);
 
   void visitNode(Node node) {
     node.visitChildren(this);
     constants.nodeConstantMap.remove(node);
   }
 }