Fix the toJson/fromJson test.

test/credentials_test.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:convert';
 
 import 'package:http/http.dart' as http;
 import 'package:oauth2/oauth2.dart' as oauth2;
 import 'package:test/test.dart';
@@ skipping 213 matching lines @@
         httpClient: httpClient);
     expect(credentials.accessToken, equals('new access token'));
     expect(credentials.refreshToken, equals('new refresh token'));
   });
 
   group("fromJson", () {
     oauth2.Credentials fromMap(Map map) =>
       new oauth2.Credentials.fromJson(JSON.encode(map));
 
     test("should load the same credentials from toJson", () {
+      // Round the expiration down to milliseconds since epoch, since that's
+      // what the credentials file stores. Otherwise sub-millisecond time gets
+      // in the way.
       var expiration = new DateTime.now().subtract(new Duration(hours: 1));
+      expiration = new DateTime.fromMillisecondsSinceEpoch(
+          expiration.millisecondsSinceEpoch);
+
       var credentials = new oauth2.Credentials(
           'access token',
           refreshToken: 'refresh token',
           tokenEndpoint: tokenEndpoint,
           scopes: ['scope1', 'scope2'],
           expiration: expiration);
       var reloaded = new oauth2.Credentials.fromJson(credentials.toJson());
 
       expect(reloaded.accessToken, equals(credentials.accessToken));
       expect(reloaded.refreshToken, equals(credentials.refreshToken));
@@ skipping 35 matching lines @@
       expect(() => fromMap({"accessToken": "foo", "scopes": 12}),
           throwsFormatException);
     });
 
     test("should throw a FormatException if expiration is not an int", () {
       expect(() => fromMap({"accessToken": "foo", "expiration": "12"}),
           throwsFormatException);
     });
   });
 }