Fix warnings_checker.dart handling of multitests

tests/language/this_test.dart

 // Copyright (c) 2012, 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 Foo {
   var x;
   f() {}
 
   testMe() {
-    x.this; /// 01: compile-time error
-    x.this(); /// 02: compile-time error
-    x.this.x; /// 03: compile-time error
-    x.this().x; /// 04: compile-time error
-    f().this; /// 05: compile-time error
-    f().this(); /// 06: compile-time error
-    f().this.f(); /// 07: compile-time error
-    f().this().f(); /// 08: compile-time error
+    x.this; //# 01: compile-time error
+    x.this(); //# 02: compile-time error
+    x.this.x; //# 03: compile-time error
+    x.this().x; //# 04: compile-time error
+    f().this; //# 05: compile-time error
+    f().this(); //# 06: compile-time error
+    f().this.f(); //# 07: compile-time error
+    f().this().f(); //# 08: compile-time error
   }
 }
 
 main() {
   new Foo().testMe();
 }