Fix warnings_checker.dart handling of multitests

tests/language_strong/prefix_unqualified_invocation_test.dart

Index: tests/language_strong/prefix_unqualified_invocation_test.dart
diff --git a/tests/language_strong/prefix_unqualified_invocation_test.dart b/tests/language_strong/prefix_unqualified_invocation_test.dart
index 020ca3e53190ea0a3c2a2b2e7e279d90b6e09f1b..d4a174c6d989cc97f1372081ad2e0bd959ac4c43 100644
--- a/tests/language_strong/prefix_unqualified_invocation_test.dart
+++ b/tests/language_strong/prefix_unqualified_invocation_test.dart
@@ -19,11 +19,11 @@ class Base {
 
 class Derived extends Base {
   void f() {
-    p(); /// 01: compile-time error
+    p(); //# 01: compile-time error
   }
 }
 
 main() {
   new Derived().f();
-  p(); /// 02: compile-time error
+  p(); //# 02: compile-time error
 }