Pub integration tests sometimes need more than 20 seconds.

dart/pkg/unittest/lib/src/simple_configuration.dart

 // Copyright (c) 2013, 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.
 
 part of unittest;
 
 // A custom failure handler for [expect] that routes expect failures
 // to the config.
 class _ExpectFailureHandler extends DefaultFailureHandler {
   final SimpleConfiguration _config;
@@ skipping 35 matching lines @@
    * [expect]. If false, failed [expect]s will not cause the test
    * to stop (other exceptions will still terminate the test).
    */
   bool stopTestOnExpectFailure = true;
 
   // If stopTestOnExpectFailure is false, we need to capture failures, which
   // we do with this List.
   final _testLogBuffer = <Pair<String, StackTrace>>[];
 
   /// How long a [TestCase] can run before it is considered an error.
+  /// A [timeout] value of [:null:] means that the limit is infinite.
   Duration timeout = const Duration(seconds: 20);
 
   /**
    * The constructor sets up a failure handler for [expect] that redirects
    * [expect] failures to [onExpectFailure].
    */
   SimpleConfiguration() : super.blank() {
     configureExpectFailureHandler(new _ExpectFailureHandler(this));
   }
 
@@ skipping 147 matching lines @@
       }
     }
   }
 
   void _postMessage(String message) {
     // In dart2js browser tests, the JavaScript-based test controller
     // intercepts calls to print and listens for "secret" messages.
     print(message);
   }
 }