Fix warnings_checker.dart handling of multitests

tests/language_strong/await_backwards_compatibility_test.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 
 import 'package:expect/expect.dart';
 
 get await => 4;
 
 // For functions that are declared with the async modifier we treat await as
 // keyword.
 
 test0() async {
   var x = await 7;
   Expect.equals(7, x);
-  var await = 1; // /// await1: compile-time error
+  var await = 1; // //# await1: compile-time error
 }
 
 test1() async {
   var x = await 9;
   Expect.equals(9, x);
-  var y = await; // /// await2: compile-time error
+  var y = await; // //# await2: compile-time error
 }
 
 // For functions that are not declared with the async modifier we allow await to
 // be used as an identifier.
 
 test2() {
   var y = await;
   Expect.equals(4, y);
-  var x = await 1; // /// await3: compile-time error
+  var x = await 1; // //# await3: compile-time error
 }
 
 test3() {
   var await = 3;
   Expect.equals(3, await);
-  var x = await 1; // /// await4: compile-time error
+  var x = await 1; // //# await4: compile-time error
 }
 
 main() {
   test0();
   test1();
   test2();
   test3();
 }