Fix chainToCompleter.

utils/tests/pub/test_pub.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.
 
 /// Test infrastructure for testing pub. Unlike typical unit tests, most pub
 /// tests are integration tests that stage some stuff on the file system, run
 /// pub, and then validate the results. This library provides an API to build
 /// tests like that.
 library test_pub;
 
@@ skipping 809 matching lines @@
         Expect.fail('No files in $dir match pattern $name.');
       }
       if (matches.length == 1) return validate(matches[0]);
 
       var failures = [];
       var successes = 0;
       var completer = new Completer();
       checkComplete() {
         if (failures.length + successes != matches.length) return;
         if (successes > 0) {
-          completer.complete(null);
+          completer.complete();
           return;
         }
 
         var error = new StringBuffer();
         error.add("No files named $name in $dir were valid:\n");
         for (var failure in failures) {
           error.add("  $failure\n");
         }
         completer.completeError(
             new ExpectException(error.toString()), stackTrace);
@@ skipping 710 matching lines @@
 /// calling [completion] is unnecessary.
 void expectLater(Future actual, matcher, {String reason,
     FailureHandler failureHandler, bool verbose: false}) {
   _schedule((_) {
     return actual.then((value) {
       expect(value, matcher, reason: reason, failureHandler: failureHandler,
           verbose: false);
     });
   });
 }