Move a bunch of src/ files into subdirectories.

lib/src/runner/engine.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.engine;
+library unittest.runner.engine;
 
 import 'dart:async';
 import 'dart:collection';
 
-import 'live_test.dart';
-import 'state.dart';
-import 'suite.dart';
-import 'utils.dart';
+import '../backend/live_test.dart';
+import '../backend/state.dart';
+import '../backend/suite.dart';
+import '../utils.dart';
 
 /// An [Engine] manages a run that encompasses multiple test suites.
 ///
 /// The current status of every test is visible via [liveTests]. [onTestStarted]
 /// can also be used to be notified when a test is about to be run.
 ///
 /// Suites will be run in the order they're provided to [new Engine]. Tests
 /// within those suites will likewise be run in the order of [Suite.tests].
 class Engine {
   /// Whether [run] has been called yet.
@@ skipping 39 matching lines @@
       return new Future.microtask(liveTest.run).then((_) => new Future(() {}));
     }).then((_) =>
         liveTests.every((liveTest) => liveTest.state.result == Result.success));
   }
 
   /// Signals that the caller is done paying attention to test results and the
   /// engine should release any resources it has allocated.
   Future close() =>
       Future.wait(liveTests.map((liveTest) => liveTest.close()));
 }