Fix all strong-mode warnings.

lib/src/authorization_code_grant.dart

 // Copyright (c) 2012, 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 'dart:async';
 
 import 'package:http/http.dart' as http;
 
 import 'client.dart';
 import 'authorization_exception.dart';
@@ skipping 222 matching lines @@
     _state = _State.finished;
 
     return await _handleAuthorizationCode(authorizationCode);
   }
 
   /// This works just like [handleAuthorizationCode], except it doesn't validate
   /// the state beforehand.
   Future<Client> _handleAuthorizationCode(String authorizationCode) async {
     var startTime = new DateTime.now();
 
-    var headers = {};
+    var headers = <String, String>{};
 
     var body = {
       "grant_type": "authorization_code",
       "code": authorizationCode,
       "redirect_uri": this._redirectEndpoint.toString()
     };
 
     if (_basicAuth && secret != null) {
       headers["Authorization"] = basicAuthHeader(identifier, secret);
     } else {
@@ skipping 42 matching lines @@
   // [AuthorizationCodeGrant.handleAuthorizationResponse] or
   // [AuthorizationCodeGrant.handleAuthorizationCode] have been called.
   static const finished = const _State("finished");
 
   final String _name;
 
   const _State(this._name);
 
   String toString() => _name;
 }