Fix warnings_checker.dart handling of multitests

tests/language/string_interpolation9_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Almost valid string interpolation syntax.
 
 main() {
   var x;
 
-  x = "$"; //   /// 1: compile-time error
-  x = "x$"; //  /// 2: compile-time error
-  x = "$x$"; // /// 3: compile-time error
-  x = "$$x"; // /// 4: compile-time error
-  x = "$ "; //  /// 5: compile-time error
+  x = "$"; //   //# 1: compile-time error
+  x = "x$"; //  //# 2: compile-time error
+  x = "$x$"; // //# 3: compile-time error
+  x = "$$x"; // //# 4: compile-time error
+  x = "$ "; //  //# 5: compile-time error
 
-  x = '$'; //   /// 6: compile-time error
-  x = 'x$'; //  /// 7: compile-time error
-  x = '$x$'; // /// 8: compile-time error
-  x = '$$x'; // /// 9: compile-time error
-  x = '$ '; //  /// 10: compile-time error
+  x = '$'; //   //# 6: compile-time error
+  x = 'x$'; //  //# 7: compile-time error
+  x = '$x$'; // //# 8: compile-time error
+  x = '$$x'; // //# 9: compile-time error
+  x = '$ '; //  //# 10: compile-time error
 
-  x = """$"""; //   /// 11: compile-time error
-  x = """x$"""; //  /// 12: compile-time error
-  x = """$x$"""; // /// 13: compile-time error
-  x = """$$x"""; // /// 14: compile-time error
-  x = """$ """; //  /// 15: compile-time error
+  x = """$"""; //   //# 11: compile-time error
+  x = """x$"""; //  //# 12: compile-time error
+  x = """$x$"""; // //# 13: compile-time error
+  x = """$$x"""; // //# 14: compile-time error
+  x = """$ """; //  //# 15: compile-time error
 
-  x = '''$'''; //   /// 16: compile-time error
-  x = '''x$'''; //  /// 17: compile-time error
-  x = '''$x$'''; // /// 18: compile-time error
-  x = '''$$x'''; // /// 19: compile-time error
-  x = '''$ '''; //  /// 20: compile-time error
+  x = '''$'''; //   //# 16: compile-time error
+  x = '''x$'''; //  //# 17: compile-time error
+  x = '''$x$'''; // //# 18: compile-time error
+  x = '''$$x'''; // //# 19: compile-time error
+  x = '''$ '''; //  //# 20: compile-time error
 
   return x;
 }