Add metadata to messages emitted by live tests.

lib/src/backend/live_test.dart

Index: lib/src/backend/live_test.dart
diff --git a/lib/src/backend/live_test.dart b/lib/src/backend/live_test.dart
index 5bea893ceb8f7e971a613e87121e75f8bbe98678..ca5a260e9de0ea49e714a4c67b9c27e7ec458083 100644
--- a/lib/src/backend/live_test.dart
+++ b/lib/src/backend/live_test.dart
@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'group.dart';
+import 'message.dart';
 import 'state.dart';
 import 'suite.dart';
 import 'test.dart';
@@ -84,8 +85,8 @@ abstract class LiveTest {
   /// will be [Chain]s.
   Stream<AsyncError> get onError;
 
-  /// A stream that emits lines printed by the test.
-  Stream<String> get onPrint;
+  /// A stream that emits messages produced by the test.
+  Stream<Message> get onMessage;
 
   /// A [Future] that completes once the test is complete.
   ///