Fix warnings_checker.dart handling of multitests

tests/lib_strong/mirrors/metadata_scope_test.dart

Index: tests/lib_strong/mirrors/metadata_scope_test.dart
diff --git a/tests/lib_strong/mirrors/metadata_scope_test.dart b/tests/lib_strong/mirrors/metadata_scope_test.dart
index 6f4a050dad389dbeb499089e6da94afa6467d0e6..6f20ea1ab76d4013bfa350d13afed69620c6d36e 100644
--- a/tests/lib_strong/mirrors/metadata_scope_test.dart
+++ b/tests/lib_strong/mirrors/metadata_scope_test.dart
@@ -15,7 +15,7 @@ class Annotation {
 
 // Note there is no compile-time constant 'foo' in scope. In particular, A.foo
 // is not in scope here.
-@Annotation(foo) // /// 01: compile-time error
+@Annotation(foo) // //# 01: compile-time error
 class A <@Annotation(foo) T> {
   @Annotation(foo)
   static foo() {}