Fix warnings_checker.dart handling of multitests

tests/compiler/dart2js_extra/23486_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.
 
 // Regression test for http://dartbug.com/23486/
 //
 // Dart2js used to crash when using `super` and prefixes inside parenthesized
 // expressions.
 import 'package:expect/expect.dart';
 
 import '23486_helper.dart' as p;
 
 class B {
   var field = 1;
 }
 
 class A extends B {
   m() {
-    (super).field = 1; /// 01: compile-time error
+    (super).field = 1; //# 01: compile-time error
   }
 }
 
 class C {
   C();
   C.name();
 }
 
 class D extends C {
   D() : super();
-  D.name() : (super).name(); /// 02: compile-time error
+  D.name() : (super).name(); //# 02: compile-time error
 }
 
 main() {
-  Expect.throws(new A().m); //       /// 01: continued
-  Expect.throws(() => new D.name()); /// 02: continued
-  Expect.throws(() => (p).x); //     /// 03: compile-time error
+  Expect.throws(new A().m); //       //# 01: continued
+  Expect.throws(() => new D.name()); //# 02: continued
+  Expect.throws(() => (p).x); //     //# 03: compile-time error
 }