Added missing entry into runtime/observatory/observatory_sources.gypi

runtime/observatory/observatory_sources.gypi

 # 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.
 
 # This file contains all dart, css, and html sources for Observatory.
 {
   'sources': [
     'lib/app.dart',
     'lib/cli.dart',
     'lib/cpu_profile.dart',
@@ skipping 167 matching lines @@
     'lib/src/mocks/objects/class.dart',
     'lib/src/mocks/objects/isolate.dart',
     'lib/src/mocks/objects/library.dart',
     'lib/src/mocks/objects/notification.dart',
     'lib/src/mocks/objects/script.dart',
     'lib/src/mocks/objects/source_location.dart',
     'lib/src/mocks/objects/target.dart',
     'lib/src/mocks/objects/vm.dart',
     'lib/src/mocks/repositories/crash_dump.dart',
     'lib/src/mocks/repositories/notification.dart',
+    'lib/src/mocks/repositories/target.dart',
     'lib/src/models/exceptions.dart',
     'lib/src/models/objects/breakpoint.dart',
     'lib/src/models/objects/class.dart',
     'lib/src/models/objects/error.dart',
     'lib/src/models/objects/event.dart',
     'lib/src/models/objects/extension_data.dart',
     'lib/src/models/objects/frame.dart',
     'lib/src/models/objects/instance.dart',
     'lib/src/models/objects/isolate.dart',
     'lib/src/models/objects/library.dart',
@@ skipping 15 matching lines @@
     'lib/utils.dart',
     'web/favicon.ico',
     'web/index.html',
     'web/main.dart',
     'web/third_party/trace_viewer_full.html',
     'web/timeline.html',
     'web/timeline.js',
     'web/timeline_message_handler.js',
   ],
 }