Fix warnings_checker.dart handling of multitests

tests/language/string_interpolation4_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.
 
 // A dollar must be followed by a "{" or an identifier.
 
 class StringInterpolation4NegativeTest {
   static testMain() {
     // Dollar not followed by "{" or identifier.
-    print("-" + "$" + "foo"); /// 01: compile-time error
+    print("-" + "$" + "foo"); //# 01: compile-time error
   }
 }
 
 main() {
   StringInterpolation4NegativeTest.testMain();
 }