Added thread.dart and zone.dart to the observatory sources to resolve build issues.

runtime/observatory/observatory_sources.gypi

 # Copyright (c) 2016, 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/allocation_profile.dart',
     'lib/app.dart',
     'lib/cli.dart',
@@ skipping 155 matching lines @@
     'lib/src/models/objects/objectstore.dart',
     'lib/src/models/objects/pc_descriptors.dart',
     'lib/src/models/objects/persistent_handles.dart',
     'lib/src/models/objects/ports.dart',
     'lib/src/models/objects/retaining_path.dart',
     'lib/src/models/objects/sample_profile.dart',
     'lib/src/models/objects/script.dart',
     'lib/src/models/objects/sentinel.dart',
     'lib/src/models/objects/source_location.dart',
     'lib/src/models/objects/target.dart',
+    'lib/src/models/objects/thread.dart',
     'lib/src/models/objects/timeline_event.dart',
     'lib/src/models/objects/token_stream.dart',
     'lib/src/models/objects/type_arguments.dart',
     'lib/src/models/objects/unknown.dart',
     'lib/src/models/objects/vm.dart',
+    'lib/src/models/objects/zone.dart',
     'lib/src/models/repositories/allocation_profile.dart',
     'lib/src/models/repositories/breakpoint.dart',
     'lib/src/models/repositories/class.dart',
     'lib/src/models/repositories/context.dart',
     'lib/src/models/repositories/eval.dart',
     'lib/src/models/repositories/event.dart',
     'lib/src/models/repositories/field.dart',
     'lib/src/models/repositories/flag.dart',
     'lib/src/models/repositories/function.dart',
     'lib/src/models/repositories/heap_snapshot.dart',
@@ skipping 57 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',
   ]
 }