Add missing gyp dependency for timeline_message_handler.js.

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',
     'lib/debugger.dart',
     'lib/elements.dart',
     'lib/elements.html',
     'lib/object_graph.dart',
     'lib/service.dart',
     'lib/service_common.dart',
+    'lib/service_html.dart',
     'lib/service_io.dart',
-    'lib/service_html.dart',
     'lib/src/app/analytics.dart',
     'lib/src/app/application.dart',
     'lib/src/app/location_manager.dart',
     'lib/src/app/page.dart',
     'lib/src/app/settings.dart',
     'lib/src/app/target_manager.dart',
     'lib/src/app/view_model.dart',
     'lib/src/cli/command.dart',
     'lib/src/cpu_profile/cpu_profile.dart',
     'lib/src/debugger/debugger.dart',
     'lib/src/debugger/debugger_location.dart',
     'lib/src/elements/action_link.dart',
     'lib/src/elements/action_link.html',
     'lib/src/elements/class_ref.dart',
     'lib/src/elements/class_ref.html',
     'lib/src/elements/class_tree.dart',
     'lib/src/elements/class_tree.html',
     'lib/src/elements/class_view.dart',
     'lib/src/elements/class_view.html',
     'lib/src/elements/code_ref.dart',
     'lib/src/elements/code_ref.html',
     'lib/src/elements/code_view.dart',
     'lib/src/elements/code_view.html',
     'lib/src/elements/context_ref.dart',
     'lib/src/elements/context_ref.html',
     'lib/src/elements/context_view.dart',
     'lib/src/elements/context_view.html',
     'lib/src/elements/cpu_profile.dart',
     'lib/src/elements/cpu_profile.html',
+    'lib/src/elements/css/shared.css',
     'lib/src/elements/curly_block.dart',
     'lib/src/elements/curly_block.html',
     'lib/src/elements/debugger.dart',
     'lib/src/elements/debugger.html',
     'lib/src/elements/error_ref.dart',
     'lib/src/elements/error_ref.html',
     'lib/src/elements/error_view.dart',
     'lib/src/elements/error_view.html',
     'lib/src/elements/eval_box.dart',
     'lib/src/elements/eval_box.html',
@@ skipping 12 matching lines @@
     'lib/src/elements/general_error.dart',
     'lib/src/elements/general_error.html',
     'lib/src/elements/heap_map.dart',
     'lib/src/elements/heap_map.html',
     'lib/src/elements/heap_profile.dart',
     'lib/src/elements/heap_profile.html',
     'lib/src/elements/heap_snapshot.dart',
     'lib/src/elements/heap_snapshot.html',
     'lib/src/elements/icdata_view.dart',
     'lib/src/elements/icdata_view.html',
+    'lib/src/elements/img/chromium_icon.png',
+    'lib/src/elements/img/dart_icon.png',
+    'lib/src/elements/img/isolate_icon.png',
     'lib/src/elements/inbound_reference.dart',
     'lib/src/elements/inbound_reference.html',
     'lib/src/elements/instance_ref.dart',
     'lib/src/elements/instance_ref.html',
     'lib/src/elements/instance_view.dart',
     'lib/src/elements/instance_view.html',
     'lib/src/elements/instructions_view.dart',
     'lib/src/elements/instructions_view.html',
     'lib/src/elements/io_view.dart',
     'lib/src/elements/io_view.html',
@@ skipping 48 matching lines @@
     'lib/src/elements/timeline_page.dart',
     'lib/src/elements/timeline_page.html',
     'lib/src/elements/view_footer.dart',
     'lib/src/elements/view_footer.html',
     'lib/src/elements/vm_connect.dart',
     'lib/src/elements/vm_connect.html',
     'lib/src/elements/vm_ref.dart',
     'lib/src/elements/vm_ref.html',
     'lib/src/elements/vm_view.dart',
     'lib/src/elements/vm_view.html',
-    'lib/src/elements/css/shared.css',
-    'lib/src/elements/img/chromium_icon.png',
-    'lib/src/elements/img/dart_icon.png',
-    'lib/src/elements/img/isolate_icon.png',
     'lib/src/service/object.dart',
     'lib/tracer.dart',
     'lib/utils.dart',
+    'web/favicon.ico',
     'web/index.html',
     'web/main.dart',
-    'web/favicon.ico',
     'web/third_party/trace_viewer_full.html',
+    'web/timeline.html',
     'web/timeline.js',
-    'web/timeline.html',
+    'web/timeline_message_handler.js',
   ],
 }