Fix all strong-mode warnings.

lib/src/resource_owner_password_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 'handle_access_token_response.dart';
@@ skipping 27 matching lines @@
     bool basicAuth: true,
     http.Client httpClient}) async {
   var startTime = new DateTime.now();
 
   var body = {
     "grant_type": "password",
     "username": username,
     "password": password
   };
 
-  var headers = {};
+  var headers = <String, String>{};
 
   if (identifier != null) {
     if (basicAuth) {
       headers['Authorization'] = basicAuthHeader(identifier, secret);
     } else {
       body['client_id'] = identifier;
       if (secret != null) body['client_secret'] = secret;
     }
   }
 
   if (scopes != null && !scopes.isEmpty) body['scope'] = scopes.join(' ');
 
   if (httpClient == null) httpClient = new http.Client();
   var response = await httpClient.post(authorizationEndpoint,
       headers: headers, body: body);
 
   var credentials = await handleAccessTokenResponse(
       response, authorizationEndpoint, startTime, scopes);
   return new Client(credentials, identifier: identifier, secret: secret);
 }