Fix warnings_checker.dart handling of multitests

tests/language/vm/debug_break_vm_test.dart

Index: tests/language/vm/debug_break_vm_test.dart
diff --git a/tests/language/vm/debug_break_vm_test.dart b/tests/language/vm/debug_break_vm_test.dart
index 379dd00f490aec03871fa7a080ddee375b2e228d..f944e8397f3586e3e620901750dfcca1b4490389 100644
--- a/tests/language/vm/debug_break_vm_test.dart
+++ b/tests/language/vm/debug_break_vm_test.dart
@@ -5,16 +5,16 @@
 // A debug break is not valid Dart syntax unless --enable-debug-break.
 
 test(i) {
-  break "outside_loop"; // /// 02: compile-time error
+  break "outside_loop"; // //# 02: compile-time error
   do {
     if (i > 15) {
-      break "inside_loop"; // /// 03: compile-time error
+      break "inside_loop"; // //# 03: compile-time error
     }
   } while (false);
 }
 
 void main() {
-  break "gdb"; //  /// 01: compile-time error
+  break "gdb"; //  //# 01: compile-time error
   for (var i = 0; i < 20; i++) {
     test(i);
   }