Fix warnings_checker.dart handling of multitests

tests/compiler/dart2js_extra/throw2_test.dart

 // Copyright (c) 2011, 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.
 
 void throw1() {
   throw "foo";
 }
 
 void throw2() {
   if (true) throw "foo";
@@ skipping 13 matching lines @@
   } else {
     throw "bar";
   }
 }
 
 void nonThrow5() {
   if (false) throw "foo";
 }
 
 void main() {
-  throw1(); // /// 01: runtime error
-  throw2(); // /// 02: runtime error
-  throw3(); // /// 03: runtime error
-  throw4(); // /// 04: runtime error
+  throw1(); // //# 01: runtime error
+  throw2(); // //# 02: runtime error
+  throw3(); // //# 03: runtime error
+  throw4(); // //# 04: runtime error
   nonThrow5();
 }