Fix warnings_checker.dart handling of multitests

tests/language_strong/not_enough_positional_arguments_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.
 
 foo(a, [b]) {
 }
 
 bar(a, {b}) {
 }
 
 class A {
   A();
   A.test(a, [b]);
 }
 
 class B {
   B()
-    : super.test(b: 1) // /// 01: runtime error
+    : super.test(b: 1) // //# 01: runtime error
   ;
 }
 
 class C extends A {
   C()
-    : super.test(b: 1) // /// 02: runtime error
+    : super.test(b: 1) // //# 02: runtime error
   ;
 }
 
 class D {
   D();
   D.test(a, {b});
 }
 
 class E extends D {
   E()
-    : super.test(b: 1) // /// 05: runtime error
+    : super.test(b: 1) // //# 05: runtime error
   ;
 }
 
 main() {
-  new A.test(b: 1); // /// 00: runtime error
+  new A.test(b: 1); // //# 00: runtime error
   new B();
   new C();
-  new D.test(b: 1); // /// 03: runtime error
+  new D.test(b: 1); // //# 03: runtime error
   new E();
-  foo(b: 1); // /// 06: runtime error
-  bar(b: 1); // /// 07: runtime error
+  foo(b: 1); // //# 06: runtime error
+  bar(b: 1); // //# 07: runtime error
 }