Fix warnings_checker.dart handling of multitests

tests/compiler/dart2js_extra/LayoutTests_fast_mediastream_getusermedia_t01_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.
 
 /// Test derived from a broken co19 test
 /// (LayoutTests/fast/mediastream/getusermedia_t01.dart). Caused dart2js to
 /// crash.
 
 foo() {}
 
 gotStream1(stream) {
   foo()
-      . /// 01: compile-time error
+      . //# 01: compile-time error
       .then();
 }
 
 void main() {
   print(gotStream1);
 }