Fix warnings_checker.dart handling of multitests

tests/language/const_constructor_mixin2_test.dart

Index: tests/language/const_constructor_mixin2_test.dart
diff --git a/tests/language/const_constructor_mixin2_test.dart b/tests/language/const_constructor_mixin2_test.dart
index 558014bf07ed00c5a5d65102d5ecfc696590a8c3..f323b5a33b9bb26267c00ba0b99f906fcb7350a8 100644
--- a/tests/language/const_constructor_mixin2_test.dart
+++ b/tests/language/const_constructor_mixin2_test.dart
@@ -11,12 +11,12 @@ class A {
 }
 
 class B extends A 
-    with Mixin // /// 01: compile-time error
+    with Mixin // //# 01: compile-time error
     {
   const B(foo) : super(foo);
 }
 
 main() {
   var a = const B(42);
-  a.nonFinalField = 54; /// 01: continued
+  a.nonFinalField = 54; //# 01: continued
 }