Fix warnings_checker.dart handling of multitests

tests/language/this_test.dart

Index: tests/language/this_test.dart
diff --git a/tests/language/this_test.dart b/tests/language/this_test.dart
index df5a599be5f60d363cd88d996e746a76210fa0fa..d66500b996d4b69d504054de09c63972105a1475 100644
--- a/tests/language/this_test.dart
+++ b/tests/language/this_test.dart
@@ -7,14 +7,14 @@ class Foo {
   f() {}
 
   testMe() {
-    x.this; /// 01: compile-time error
-    x.this(); /// 02: compile-time error
-    x.this.x; /// 03: compile-time error
-    x.this().x; /// 04: compile-time error
-    f().this; /// 05: compile-time error
-    f().this(); /// 06: compile-time error
-    f().this.f(); /// 07: compile-time error
-    f().this().f(); /// 08: compile-time error
+    x.this; //# 01: compile-time error
+    x.this(); //# 02: compile-time error
+    x.this.x; //# 03: compile-time error
+    x.this().x; //# 04: compile-time error
+    f().this; //# 05: compile-time error
+    f().this(); //# 06: compile-time error
+    f().this.f(); //# 07: compile-time error
+    f().this().f(); //# 08: compile-time error
   }
 }