Reorganize barback's source files.

pkg/barback/test/multiset_test.dart

 // Copyright (c) 2013, 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.
 
 library barback.test.multiset_test;
 
-import 'package:barback/src/multiset.dart';
+import 'package:barback/src/utils/multiset.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 main() {
   initConfig();
 
   test("new Multiset() creates an empty set", () {
     var multiSet = new Multiset();
     expect(multiSet, isEmpty);
@@ skipping 52 matching lines @@
   test("iterator orders distinct elements in insertion order", () {
     var multiSet = new Multiset()..add(1)..add(2)..add(3)..add(4)..add(5);
     expect(multiSet.toList(), equals([1, 2, 3, 4, 5]));
   });
 
   test("iterator groups multiple copies of an element together", () {
     var multiSet = new Multiset()..add(1)..add(2)..add(1)..add(2)..add(1);
     expect(multiSet.toList(), equals([1, 1, 1, 2, 2]));
   });
 }