Move a bunch of src/ files into subdirectories.

test/expect_async_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 'package:unittest/src/state.dart';
+import 'package:unittest/src/backend/state.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
   group("supports a function with this many arguments:", () {
     test("0", () {
       var callbackRun = false;
       return runTest(() {
         expectAsync(() {
@@ skipping 303 matching lines @@
           caughtError = true;
         }
       }).then((liveTest) {
         expectTestFailed(liveTest, 'oh no');
         expect(returnValue, isNull);
         expect(caughtError, isFalse);
       });
     });
   });
 }