Fix warnings_checker.dart handling of multitests

tests/language/compile_time_constant13_test.dart

 // Copyright (c) 2014, 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.
 
 class A {
-  final x; //      /// 01: ok
+  final x; //      //# 01: ok
                  /// 02: compile-time error
-  var x; //        /// 03: compile-time error
-  get x => null; /// 04: compile-time error
-  set x(v) {} //   /// 05: compile-time error
+  var x; //        //# 03: compile-time error
+  get x => null; //# 04: compile-time error
+  set x(v) {} //   //# 05: compile-time error
 
   const A()
-    : x = 'foo' // /// 01: continued
-    : x = 'foo' // /// 02: continued
-    : x = 'foo' // /// 03: continued
-    : x = 'foo' // /// 04: continued
-    : x = 'foo' // /// 05: continued
+    : x = 'foo' // //# 01: continued
+    : x = 'foo' // //# 02: continued
+    : x = 'foo' // //# 03: continued
+    : x = 'foo' // //# 04: continued
+    : x = 'foo' // //# 05: continued
   ;
 }
 
 use(x) => x;
 
 A a = const A();
 
 main() {
   use(a);
 }