Add metadata to messages emitted by live tests.

test/runner/browser/loader_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 @Tags(const ["chrome"])
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 146 matching lines @@
 void main() {
   print('print within test');
 }
 """);
     var suites = await _loader.loadFile(p.join(_sandbox, 'a_test.dart'))
         .toList();
     expect(suites, hasLength(1));
     var loadSuite = suites.first;
 
     var liveTest = await loadSuite.group.entries.single.load(loadSuite);
-    expect(liveTest.onPrint.first, completion(equals("print within test")));
+    expect(liveTest.onMessage.first.then((message) => message.text),
+        completion(equals("print within test")));
     await liveTest.run();
     expectTestPassed(liveTest);
   });
 }