Fix warnings_checker.dart handling of multitests

tests/language_strong/implicit_this_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.
 
 import "package:expect/expect.dart";
 
 abstract class Interface {
   final x;
 }
 
 abstract class Abstract implements Interface {
   String toString() => x.toString();
 }
 
 // This class does not implement "x" either, but it is not marked
 // abstract.
-class SubAbstract1 extends Abstract { } /// 01: static type warning
+class SubAbstract1 extends Abstract { } //# 01: static type warning
 
 // This class is implicitly abstract as it declares an abstract getter
 // method.
 class SubAbstract2 extends Abstract {
   get x;  // Abstract.
 }
 
 // This class does not implement "x" either, but it is not marked
 // abstract.
-class SubSubAbstract2 extends SubAbstract2 { } /// 04: static type warning
+class SubSubAbstract2 extends SubAbstract2 { } //# 04: static type warning
 
 class Concrete extends Abstract {
   get x => 7;
 }
 
 class SubConcrete extends Concrete {
   final x;
   SubConcrete(this.x);
 }
 
 void main() {
-  var x = new Abstract(); /// 02: runtime error
-  var y = new SubAbstract1(); /// 01: continued
+  var x = new Abstract(); //# 02: runtime error
+  var y = new SubAbstract1(); //# 01: continued
   var z = new SubAbstract2();
-  var a = new SubSubAbstract2(); /// 04: continued
-  Expect.equals(x, x); /// 02: continued
+  var a = new SubSubAbstract2(); //# 04: continued
+  Expect.equals(x, x); //# 02: continued
   Expect.equals('7', new Concrete().toString());
   Expect.equals('42', new SubConcrete(42).toString());
   Expect.equals('7', new SubConcrete(new Concrete()).toString());
 }