pkg/mock: restructure library layout

pkg/mock/lib/src/responder.dart

+// Copyright (c) 2014, 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 mock.responder;
+
+import 'action.dart';
+
+/**
+ * The behavior of a method call in the mock library is specified
+ * with [Responder]s. A [Responder] has a [value] to throw
+ * or return (depending on the type of [action]),
+ * and can either be one-shot, multi-shot, or infinitely repeating,
+ * depending on the value of [count (1, greater than 1, or 0 respectively).
+ */
+class Responder {
+  final Object value;
+  final Action action;
+  int count;
+  Responder(this.value, [this.count = 1, this.action = Action.RETURN]);
+}