Incremental compiler: support optional arguments.

dart/tests/try/web/sandbox.dart

 // Copyright (c) 2014, 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.
 
 /// Helper library that creates an iframe sandbox that can be used to load
 /// code.
 library trydart.test.sandbox;
 
 import 'dart:html';
 import 'dart:async';
@@ skipping 84 matching lines @@
       switch (message) {
         case 'dart-calling-main':
         case 'dart-main-done':
         case 'unittest-suite-done':
         case 'unittest-suite-fail':
         case 'unittest-suite-success':
         case 'unittest-suite-wait-for-done':
           break;
 
         default:
-          completer.completeError('Unexpected message: "$message".');
+          completer.completeError(
+              'Unexpected message: "$message" (expected "$expectedMessage").');
       }
     }
   }
 
   Future expect(data) {
     if (data is String) {
       Expect.isTrue(completer == null || completer.isCompleted);
       expectedMessage = data;
       completer = new Completer();
       return completer.future;
     } else if (data is Iterable) {
       return Future.forEach(data, expect);
     } else {
       throw 'Unexpected data type: ${data.runtimeType}.';
     }
   }
 
   void start() {
     wallclock = new Stopwatch()..start();
     window.onMessage.listen(onMessage);
   }
 }