Fix all strong-mode warnings.

lib/src/credentials.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 'dart:collection';
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
 
@@ skipping 114 matching lines @@
     if (expiration != null) {
       validate(expiration is int,
           'field "expiration" was not an int, was "$expiration"');
       expiration = new DateTime.fromMillisecondsSinceEpoch(expiration);
     }
 
     return new Credentials(
         parsed['accessToken'],
         refreshToken: parsed['refreshToken'],
         tokenEndpoint: tokenEndpoint,
-        scopes: scopes,
+        scopes: (scopes as List).map((scope) => scope as String),
         expiration: expiration);
   }
 
   /// Serializes a set of credentials to JSON.
   ///
   /// Nothing is guaranteed about the output except that it's valid JSON and
   /// compatible with [Credentials.toJson].
   String toJson() => JSON.encode({
     'accessToken': accessToken,
     'refreshToken': refreshToken,
@@ skipping 31 matching lines @@
 
     var startTime = new DateTime.now();
     if (refreshToken == null) {
       throw new StateError("Can't refresh credentials without a refresh "
           "token.");
     } else if (tokenEndpoint == null) {
       throw new StateError("Can't refresh credentials without a token "
           "endpoint.");
     }
 
-    var headers = {};
+    var headers = <String, String>{};
 
     var body = {
       "grant_type": "refresh_token",
       "refresh_token": refreshToken
     };
     if (!scopes.isEmpty) body["scope"] = scopes.join(' ');
 
     if (basicAuth && secret != null) {
       headers["Authorization"] = basicAuthHeader(identifier, secret);
     } else {
@@ skipping 10 matching lines @@
     // we should re-use the one we already have.
     if (credentials.refreshToken != null) return credentials;
     return new Credentials(
         credentials.accessToken,
         refreshToken: this.refreshToken,
         tokenEndpoint: credentials.tokenEndpoint,
         scopes: credentials.scopes,
         expiration: credentials.expiration);
   }
 }