Move a bunch of src/ files into subdirectories.

lib/src/backend/suite.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.
 
-library unittest.suite;
+library unittest.backend.suite;
 
 import 'dart:collection';
 
 import 'test.dart';
 
 /// A test suite.
 ///
 /// A test suite is a set of tests that are intended to be run together and that
 /// share default configuration.
 class Suite {
   /// The name of the test suite.
   final String name;
 
   /// The tests in the test suite.
   final List<Test> tests;
 
   Suite(this.name, Iterable<Test> tests)
       : tests = new UnmodifiableListView<Test>(tests.toList());
 }