Fix warnings_checker.dart handling of multitests

tests/lib_strong/mirrors/metadata_constructor_arguments_test.dart

 // compile options: --emit-metadata
 // Copyright (c) 2013, 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 Issue 13817.
 
 library test.metadata_constructor_arguments;
 
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class Tag {
   final name;
   const Tag({named}) : this.name = named;
 }
 
-@Tag(named: undefined) // /// 01: compile-time error
+@Tag(named: undefined) // //# 01: compile-time error
 class A {}
 
 @Tag(named: 'valid')
 class B {}
 
 @Tag(named: C.STATIC_FIELD)
 class C {
   static const STATIC_FIELD = 3;
 }
 
-@Tag(named: D.instanceMethod()) // /// 02: compile-time error
+@Tag(named: D.instanceMethod()) // //# 02: compile-time error
 class D {
   instanceMethod() {}
 }
 
-@Tag(named: instanceField) // /// 03: compile-time error
+@Tag(named: instanceField) // //# 03: compile-time error
 class E {
   var instanceField;
 }
 
-@Tag(named: F.nonConstStaticField) // /// 04: compile-time error
+@Tag(named: F.nonConstStaticField) // //# 04: compile-time error
 class F {
   static var nonConstStaticField = 6;
 }
 
-@Tag(named: instanceMethod) // /// 05: compile-time error
+@Tag(named: instanceMethod) // //# 05: compile-time error
 class G {
   instanceMethod() {}
 }
 
-@Tag(named: this) // /// 06: compile-time error
+@Tag(named: this) // //# 06: compile-time error
 class H {
   instanceMethod() {}
 }
 
-@Tag(named: super) // /// 07: compile-time error
+@Tag(named: super) // //# 07: compile-time error
 class I {
   instanceMethod() {}
 }
 
 checkMetadata(DeclarationMirror mirror, List expectedMetadata) {
   Expect.listEquals(expectedMetadata.map(reflect).toList(), mirror.metadata);
 }
 
 main() {
   reflectClass(A).metadata;
   checkMetadata(reflectClass(B), [const Tag(named: 'valid')]);
   checkMetadata(reflectClass(C), [const Tag(named: C.STATIC_FIELD)]);
   reflectClass(D).metadata;
   reflectClass(E).metadata;
   reflectClass(F).metadata;
   reflectClass(G).metadata;
   reflectClass(H).metadata;
   reflectClass(I).metadata;
 }