Add metadata to messages emitted by live tests.

test/runner/browser/loader_test.dart

Index: test/runner/browser/loader_test.dart
diff --git a/test/runner/browser/loader_test.dart b/test/runner/browser/loader_test.dart
index 41f10391aecdb18ed1a893bd030f461b64248500..1c924d3e4c0f6c7e90621fab23b3c34fd75cbdb9 100644
--- a/test/runner/browser/loader_test.dart
+++ b/test/runner/browser/loader_test.dart
@@ -164,7 +164,8 @@ void main() {
     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);
   });