Fix warnings_checker.dart handling of multitests

tests/language/mixin_supertype_subclass3_test.dart

 // Copyright (c) 2016, 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.
 // SharedOptions=--supermixin
 
 // Validate the following test from section 12 ("Mixins") of the spec:
 //
 //     "Let M_A be a mixin derived from a class M with direct superclass
 //     S_static.
 //
 //     Let A be an application of M_A.  It is a static warning if the
 //     superclass of A is not a subtype of S_static."
 
 // In this test, M is declared as `class M = S_static with G;`.
 
 class B {}
 class C {}
 class D {}
 class E extends B with C implements D {}
 class F extends E {}
 class A
-  = E with M; class M = B with G; class G /// 01: ok
-  = E with M; class M = C with G; class G /// 02: static type warning
-  = E with M; class M = D with G; class G /// 03: ok
-  = E with M; class M = E with G; class G /// 04: ok
-  = E with M; class M = F with G; class G /// 05: static type warning
+  = E with M; class M = B with G; class G //# 01: ok
+  = E with M; class M = C with G; class G //# 02: static type warning
+  = E with M; class M = D with G; class G //# 03: ok
+  = E with M; class M = E with G; class G //# 04: ok
+  = E with M; class M = F with G; class G //# 05: static type warning
   {}
 
 main() {
   new A();
 }