Move a bunch of src/ files into subdirectories.

test/invoker_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.
 
 import 'dart:async';
 
 import 'package:fake_async/fake_async.dart';
-import 'package:unittest/src/invoker.dart';
-import 'package:unittest/src/state.dart';
-import 'package:unittest/src/suite.dart';
+import 'package:unittest/src/backend/invoker.dart';
+import 'package:unittest/src/backend/state.dart';
+import 'package:unittest/src/backend/suite.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
   var suite;
   setUp(() {
     lastState = null;
     suite = new Suite("suite", []);
   });
@@ skipping 465 matching lines @@
       expectErrors(liveTest, [(error) {
         expect(lastState.status, equals(Status.complete));
         expect(error, new isInstanceOf<TimeoutException>());
       }]);
 
       liveTest.run();
       async.elapse(new Duration(seconds: 30));
     });
   });
 }