Fix warnings_checker.dart handling of multitests

tests/corelib/symbol_test.dart

Index: tests/corelib/symbol_test.dart
diff --git a/tests/corelib/symbol_test.dart b/tests/corelib/symbol_test.dart
index 0bdb09463416e64b2fbb1f75d2cc1cbed66f7d19..6b97092a670c334d7ac6787b3c07f269dccbb4d1 100644
--- a/tests/corelib/symbol_test.dart
+++ b/tests/corelib/symbol_test.dart
@@ -9,7 +9,7 @@ main() {
   print(x = const Symbol('fisk'));
 
   try {
-    print(const Symbol(0)); /// 01: compile-time error
+    print(const Symbol(0)); //# 01: compile-time error
   } on NoSuchMethodError {
     print('Caught NoSuchMethodError');
   } on TypeError {
@@ -17,13 +17,13 @@ main() {
   }
 
   try {
-    print(const Symbol('0')); /// 02: compile-time error
+    print(const Symbol('0')); //# 02: compile-time error
   } on ArgumentError catch (e) {
     print('Caught $e');
   }
 
   try {
-    print(const Symbol('_')); /// 03: compile-time error
+    print(const Symbol('_')); //# 03: compile-time error
   } on ArgumentError catch (e) {
     print('Caught $e');
   }