Fix warnings_checker.dart handling of multitests

tests/language/prefix_identifier_reference_test.dart

 // Copyright (c) 2015, 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.
 
 // Validate the following spec text from section 16.32 (Identifier Reference):
 //     Evaluation of an identifier expression e of the form id proceeds as
 //   follows:
 //     Let d be the innermost declaration in the enclosing lexical scope whose
 //   name is id or id=.  If no such declaration exists in the lexical scope,
 //   d be the declaration of the inherited member named id if it exists.
 //     - If d is a prefix p, a compile-time error occurs unless the token
 //       immediately following d is '.'.
 
 import "package:expect/expect.dart";
 import "empty_library.dart" as p;
 
 void f(x) {}
 
 main() {
-  f(p); //        /// 01: compile-time error
-  var x = p; //   /// 02: compile-time error
-  var x = p[0]; /// 03: compile-time error
-  p[0] = null; // /// 04: compile-time error
-  p += 0; //      /// 05: compile-time error
+  f(p); //        //# 01: compile-time error
+  var x = p; //   //# 02: compile-time error
+  var x = p[0]; //# 03: compile-time error
+  p[0] = null; // //# 04: compile-time error
+  p += 0; //      //# 05: compile-time error
 }