Move a bunch of src/ files into subdirectories.

lib/src/backend/state.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.state;
+library unittest.backend.state;
 
 /// The state of a [LiveTest].
 ///
 /// A test's state is made up of two components, its [status] and its [result].
 /// The [status] represents where the test is in its process of running; the
 /// [result] represents the outcome as far as its known.
 class State {
   /// Where the test is in its process of running.
   final Status status;
 
@@ skipping 81 matching lines @@
       case "error": return Result.error;
       default:
         throw new ArgumentError('Invalid result name "$name".');
     }
   }
 
   const Result._(this.name);
 
   String toString() => name;
 }