pkg/mock: restructure library layout

pkg/mock/lib/src/action.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.action;
+
+/** The ways in which a call to a mock method can be handled. */
+class Action {
+  /** Do nothing (void method) */
+  static const IGNORE = const Action._('IGNORE');
+
+  /** Return a supplied value. */
+  static const RETURN = const Action._('RETURN');
+
+  /** Throw a supplied value. */
+  static const THROW = const Action._('THROW');
+
+  /** Call a supplied function. */
+  static const PROXY = const Action._('PROXY');
+
+  const Action._(this.name);
+
+  final String name;
+
+  String toString() => 'Action: $name';
+}