Fix warnings_checker.dart handling of multitests

tests/language_strong/cascade_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 
 // Test cascades.
 
 class A {
   int x;
@@ skipping 47 matching lines @@
    ..x = 4..y = 9..check(4, 9)
    ..setX(10)..check(10, 9)
    ..y = 5..check(10, 5)
    ..swap()()()..check(5, 10)
    ..[0] = 2..check(2, 10)
    ..setX(10).setY(3)..check(10, 3)
    ..setX(7)["swap"]()..check(3, 7)
    ..import()..check(4, 7)
    ..["swap"]()()()..check(7, 4);
   a.check(7,4);
-  a..(42); // /// 01: compile-time error
-  a..37; // /// 02: compile-time error
-  a.."foo"; // /// 03: compile-time error
+  a..(42); // //# 01: compile-time error
+  a..37; // //# 02: compile-time error
+  a.."foo"; // //# 03: compile-time error
 }