Add metadata to messages emitted by live tests.

test/runner/loader_test.dart

Index: test/runner/loader_test.dart
diff --git a/test/runner/loader_test.dart b/test/runner/loader_test.dart
index ab971f0889d7df3a677d9ca3e0536984730abe19..f93bae9c24b11709d831789283c6fb3fb046a24d 100644
--- a/test/runner/loader_test.dart
+++ b/test/runner/loader_test.dart
@@ -153,7 +153,8 @@ 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);
   });