Fix warnings_checker.dart handling of multitests

tests/corelib/throw_half_surrogate_pair_test.dart

Index: tests/corelib/throw_half_surrogate_pair_test.dart
diff --git a/tests/corelib/throw_half_surrogate_pair_test.dart b/tests/corelib/throw_half_surrogate_pair_test.dart
index 8e0f98a1ff104e273fff1394dbd13b11c9ae3d73..a3545b2e3a8f9782fb208f380c67b95b3c7abd78 100644
--- a/tests/corelib/throw_half_surrogate_pair_test.dart
+++ b/tests/corelib/throw_half_surrogate_pair_test.dart
@@ -7,6 +7,6 @@ main() {
   if (trebleClef.length != 2) throw "String should be a surrogate pair";
   // These uncaught exceptions should not caush the VM to crash attempting to
   // print a malformed string.
-  throw trebleClef[0]; // /// 01: runtime error
-  throw trebleClef[1]; // /// 02: runtime error
+  throw trebleClef[0]; // //# 01: runtime error
+  throw trebleClef[1]; // //# 02: runtime error
 }