Fix warnings_checker.dart handling of multitests

tests/language_strong/variable_declaration_metadata_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.
 
 // Verify that the individual variable declarations inside a variable
 // declaration list are not allowed to be annotated with metadata.
 
 const annotation = null;
 
 var
-  @annotation /// 01: compile-time error
+  @annotation //# 01: compile-time error
   v1,
-  @annotation /// 02: compile-time error
+  @annotation //# 02: compile-time error
   v2;
 
 int
-  @annotation /// 03: compile-time error
+  @annotation //# 03: compile-time error
   v3,
-  @annotation /// 04: compile-time error
+  @annotation //# 04: compile-time error
   v4;
 
 class C {
   var
-    @annotation /// 05: compile-time error
+    @annotation //# 05: compile-time error
     f1,
-    @annotation /// 06: compile-time error
+    @annotation //# 06: compile-time error
     f2;
 
   int
-    @annotation /// 07: compile-time error
+    @annotation //# 07: compile-time error
     f3,
-    @annotation /// 08: compile-time error
+    @annotation //# 08: compile-time error
     f4;
 }
 
 use(x) => x;
 
 main() {
   use(v1);
   use(v2);
   use(v3);
   use(v4);
 
   C c = new C();
   use(c.f1);
   use(c.f2);
   use(c.f3);
   use(c.f4);
 
   var
-    @annotation /// 09: compile-time error
+    @annotation //# 09: compile-time error
     l1,
-    @annotation /// 10: compile-time error
+    @annotation //# 10: compile-time error
     l2;
 
   int
-    @annotation /// 11: compile-time error
+    @annotation //# 11: compile-time error
     l3,
-    @annotation /// 12: compile-time error
+    @annotation //# 12: compile-time error
     l4;
 
   use(l1);
   use(l2);
   use(l3);
   use(l4);
 
   for (var
-         @annotation /// 13: compile-time error
+         @annotation //# 13: compile-time error
          i1 = 0,
-         @annotation /// 14: compile-time error
+         @annotation //# 14: compile-time error
          i2 = 0; ; ) {
     use(i1);
     use(i2);
     break;
   }
 
   for (int
-         @annotation /// 15: compile-time error
+         @annotation //# 15: compile-time error
          i3 = 0,
-         @annotation /// 16: compile-time error
+         @annotation //# 16: compile-time error
          i4 = 0; ; ) {
     use(i3);
     use(i4);
     break;
   }
 }