Fix warnings_checker.dart handling of multitests

tests/language/prefix_assignment_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 that assignment to a prefix is handled consistently with the
 // following spec text from section 16.19 (Assignment):
 //     Evaluation of an assignment a of the form v = e proceeds as follows:
 //     Let d be the innermost declaration whose name is v or v=, if it exists.
 //     It is a compile-time error if d denotes a prefix object.
 
 import "empty_library.dart" as p;
 
 class Base {
   var p;
 }
 
 class Derived extends Base {
   void f() {
-    p = 1; /// 01: compile-time error
+    p = 1; //# 01: compile-time error
   }
 }
 
 main() {
   new Derived().f();
-  p = 1; /// 02: compile-time error
+  p = 1; //# 02: compile-time error
 }