Fix warnings_checker.dart handling of multitests

tests/language_strong/bad_raw_string_test.dart

 // Copyright (c) 2016, 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.
 
 main() {
   // Raw String may not contain newline (may not be multi-line).
   String x = ''
-    r' // /// 01: compile-time error
-' //      /// 01: continued
-    r" // /// 02: compile-time error
-" //      /// 02: continued
+    r' // //# 01: compile-time error
+' //      //# 01: continued
+    r" // //# 02: compile-time error
+" //      //# 02: continued
     // Test that a raw string containing just one character, a \n char, fails.
     // Enclose the test string in a bigger multiline string, except in case 03:
-    ''' // /// 03: compile-time error
+    ''' // //# 03: compile-time error
     """
-    ''' // /// 03: continued
+    ''' // //# 03: continued
     r'
 '
-    ''' // /// 03: continued
+    ''' // //# 03: continued
     """
-    ''' // /// 03: continued
+    ''' // //# 03: continued
   ;
 }