Fix warnings_checker.dart handling of multitests

tests/language/compile_time_constant_checked4_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;
   const A.a1(
-    String /// 01: checked mode compile-time error, static type warning
+    String //# 01: checked mode compile-time error, static type warning
     x) : this.a2(x);
   const A.a2(
-    String /// 02: checked mode compile-time error
+    String //# 02: checked mode compile-time error
     x) : this.a3(x);
   const A.a3(
-    String /// 03: checked mode compile-time error
+    String //# 03: checked mode compile-time error
     x) : _x = x;
 }
 
 use(x) => x;
 
 main() {
   use(const A.a1(0));
 }