Fix warnings_checker.dart handling of multitests

tests/language_strong/prefix_assignment_test.dart

Index: tests/language_strong/prefix_assignment_test.dart
diff --git a/tests/language_strong/prefix_assignment_test.dart b/tests/language_strong/prefix_assignment_test.dart
index b473fc2014ba34e00f4b32329493461ae03fdb90..14e6f51a0f0d22c4c4ab3ce64c7461931171ea75 100644
--- a/tests/language_strong/prefix_assignment_test.dart
+++ b/tests/language_strong/prefix_assignment_test.dart
@@ -16,11 +16,11 @@ class Base {
 
 class Derived extends Base {
   void f() {
-    p = 1; /// 01: compile-time error
+    p = 1; //# 01: compile-time error
   }
 }
 
 main() {
   new Derived().f();
-  p = 1; /// 02: compile-time error
+  p = 1; //# 02: compile-time error
 }