Fix warnings_checker.dart handling of multitests

tests/language/library_ambiguous_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.
 
 // This test should fail to load because we are importing two libraries
 // which define the same top level name foo, and we are referring to the name.
 
 import "library1.dart";  // Defines top level variable 'foo'
 import "library2.dart";  // Defines top level variable 'foo'
 
 class X
-extends baw //  /// 05: compile-time error
+extends baw //  //# 05: compile-time error
 {}
 
 main() {
-  print(foo); // /// 00: runtime error
-  print(bar()); // /// 01: runtime error
-  print(baz()); // /// 02: runtime error
-  print(bay()); // /// 03: runtime error
-  print(main is bax); // /// 04: static type warning, runtime error
-  var x = new X(); // /// 05: continued
+  print(foo); // //# 00: runtime error
+  print(bar()); // //# 01: runtime error
+  print(baz()); // //# 02: runtime error
+  print(bay()); // //# 03: runtime error
+  print(main is bax); // //# 04: static type warning, runtime error
+  var x = new X(); // //# 05: continued
   print("No error expected if ambiguous definitions are not used.");
 }