All strong mode cleaning of dart2js.

pkg/compiler/lib/src/serialization/element_serialization.dart

 // Copyright (c) 2015, 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.serialization.elements;
 
 import '../common.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
@@ skipping 84 matching lines @@
 ];
 
 /// Interface for a function that can serialize a set of element kinds.
 abstract class ElementSerializer {
   /// Returns the [SerializedElementKind] for [element] if this serializer
   /// supports serialization of [element] or `null` otherwise.
   SerializedElementKind getSerializedKind(Element element);
 
   /// Serializes [element] into the [encoder] using the [kind] computed
   /// by [getSerializedKind].
-  void serialize(
-      Element element, ObjectEncoder encoder, SerializedElementKind kind);
+  void serialize(covariant Element element, ObjectEncoder encoder,
+      SerializedElementKind kind);
 }
 
 class SerializerUtil {
   /// Serialize the declared members of [element] into [encoder].
   static void serializeMembers(
       Iterable<Element> members, ObjectEncoder encoder) {
     MapEncoder mapEncoder = encoder.createMap(Key.MEMBERS);
     for (Element member in members) {
       String name = member.name;
       if (member.isSetter) {
@@ skipping 832 matching lines @@
             enclosingElement, existingElement, newElement);
       case SerializedElementKind.EXTERNAL_LIBRARY:
       case SerializedElementKind.EXTERNAL_LIBRARY_MEMBER:
       case SerializedElementKind.EXTERNAL_CLASS_MEMBER:
       case SerializedElementKind.EXTERNAL_CONSTRUCTOR:
         break;
     }
     throw new UnsupportedError("Unexpected element kind '${elementKind}.");
   }
 }