[test.dart] Complain if there is non-utf8 formatted data in test output

tests/standalone/io/console_unicode_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:convert';
 import 'dart:io';
 
 main() {
+  String canary = "Canary";
   String spades = "These are three black spades: ♠♠♠";
   String german = "German characters: aäbcdefghijklmnoöpqrsßtuüvwxyz";
 
+  stdout.writeln(canary);
   stdout.writeln(spades);
   stdout.writeln(german);
   print(spades);
   print(german);
 
-  stdout.add(spades.runes.toList());
-  stdout.writeln();
-  stdout.add(german.runes.toList());
+  stdout.add(canary.runes.toList());
   stdout.writeln();
 
-  stdout.add(spades.codeUnits);
-  stdout.writeln();
-  stdout.add(german.codeUnits);
-  stdout.writeln();
-
+  stdout.writeln(canary);
   stdout.writeln(spades);
   stdout.writeln(german);
   print(spades);
   print(german);
-}
+
+  stdout.add(canary.codeUnits);
+  stdout.writeln();
+}