Fix warnings_checker.dart handling of multitests

tests/html/js_typed_interop_default_arg_test.dart

 // Copyright (c) 2015, 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.
 
 @JS()
 library js_typed_interop_test;
 
 import 'dart:html';
 
 import 'package:expect/expect.dart' show NoInline;
 import 'package:js/js.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 _injectJs() {
   document.body.append(new ScriptElement()
     ..type = 'text/javascript'
     ..innerHtml = r"""
   var Foo = {
     get42: function(b) { return arguments.length >= 1 ? b : 42; },
     get43: function(b) { return arguments.length >= 1 ? b : 43; }
   };
 """);
 }
 
 @JS()
 class Foo {
   // Note: it's invalid to provide a default value.
   external static num get42([num b
-      = 3 // /// default_value: compile-time error
+      = 3 // //# default_value: compile-time error
   ]);
   external static num get43([num b]);
 }
 
 main() {
   _injectJs();
   useHtmlConfiguration();
 
   test('call directly from dart', () {
     expect(Foo.get42(2), 2);
     expect(Foo.get42(), 42);
   });
 
   test('call tearoff from dart with arg', () {
     var f = Foo.get42;
-    expect(f(2), 2); /// explicit_argument: ok
+    expect(f(2), 2); //# explicit_argument: ok
   });
 
   test('call tearoff from dart with default', () {
     var f = Foo.get42;
     // Note: today both SSA and CPS remove the extra argument on static calls,
     // but they fail to do so on tearoffs.
-    expect(f(), 3); /// default_value: continued
+    expect(f(), 3); //# default_value: continued
 
     f = Foo.get43;
     expect(f(), 43);
   });
 }