Move a bunch of src/ files into subdirectories.

lib/src/backend/live_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.
 
-library unittest.live_test;
+library unittest.backend.live_test;
 
 import 'dart:async';
 
 import 'state.dart';
 import 'suite.dart';
 import 'test.dart';
 
 /// A runnable instance of a test.
 ///
 /// This is distinct from [Test] in order to keep [Test]. Running a test
@@ skipping 92 matching lines @@
   /// Once [close] is called, [onComplete] will complete if it hasn't already
   /// and [onStateChange] and [onError] will close immediately. This means that,
   /// if the test was running at the time [close] is called, it will never emit
   /// a [Status.complete] state-change event.
   ///
   /// This doesn't automatically happen after the test completes because there
   /// may be more asynchronous work going on in the background that could
   /// produce new errors.
   Future close();
 }