All strong mode cleaning of dart2js.

pkg/compiler/lib/src/resolution/send_structure.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.resolution.send_structure;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../elements/elements.dart';
 import '../elements/operators.dart';
 import '../elements/resolution_types.dart';
 import '../resolution/tree_elements.dart' show TreeElements;
 import '../tree/tree.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector;
 import 'access_semantics.dart';
 import 'semantic_visitor.dart';
 
 /// Interface for the structure of the semantics of a [Send] or [NewExpression]
 /// node.
 abstract class SemanticSendStructure<R, A> {
   /// Calls the matching visit method on [visitor] with [node] and [arg].
-  R dispatch(SemanticSendVisitor<R, A> visitor, Node node, A arg);
+  R dispatch(SemanticSendVisitor<R, A> visitor, covariant Node node, A arg);
 }
 
 enum SendStructureKind {
   IF_NULL,
   LOGICAL_AND,
   LOGICAL_OR,
   IS,
   IS_NOT,
   AS,
   INVOKE,
@@ skipping 2481 matching lines @@
   ThisConstructorInvokeStructure(
       this.node, this.constructor, this.callStructure);
 
   R dispatch(SemanticDeclarationVisitor<R, A> visitor, A arg) {
     return visitor.visitThisConstructorInvoke(
         node, constructor, node.argumentsNode, callStructure, arg);
   }
 
   bool get isConstructorInvoke => true;
 }