Fix unnecessary Observatory rebuilds in Flutter

runtime/observatory/observatory.gypi

 # 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.
 
 {
   'variables': {
     'gen_source_dir': '<(SHARED_INTERMEDIATE_DIR)',
   },
   'targets': [
     {
       'target_name': 'fetch_observatory_deps',
       'type': 'none',
       'dependencies': [
         'dart_bootstrap#host',
         # We use packages for building
         '../pkg/pkg.gyp:pkg_packages#target',
       ],
       'toolsets': ['host'],
       'actions': [
         {
           'action_name': 'get_obsevatory_dependencies',
           'inputs': [
             '../../tools/observatory_tool.py',
             '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
             'pubspec.yaml',
           ],
           'outputs': [
-            'pubspec.lock'
+            '<(gen_source_dir)/observatory_packages.stamp'
           ],
           'action': [
             'python',
             '../tools/observatory_tool.py',
             '--sdk=True',
             '--package-root', '<(PRODUCT_DIR)/packages',
+            '--stamp',
+            '<(gen_source_dir)/observatory_packages.stamp',
             '--dart-executable',
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart_bootstrap<(EXECUTABLE_SUFFIX)',
             '--directory', 'observatory',
             '--command', 'get',
           ],
         }
       ],
     },
     {
       'target_name': 'build_observatory',
       'type': 'none',
       'dependencies': [
         'dart_bootstrap#host',
         'fetch_observatory_deps#host',
       ],
       'toolsets': ['host'],
       'includes': [
         'observatory_sources.gypi',
       ],
       'actions': [
         {
           'action_name': 'pub_build_observatory',
           'inputs': [
             '../../tools/observatory_tool.py',
-            'pubspec.lock',
+            '<(gen_source_dir)/observatory_packages.stamp',
             '<@(_sources)',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/observatory/build/web/index.html',
           ],
           'action': [
             'python',
             '../tools/observatory_tool.py',
             '--sdk=True',
             '--package-root', '<(PRODUCT_DIR)/packages',
@@ skipping 21 matching lines @@
             '--dart-executable',
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart_bootstrap<(EXECUTABLE_SUFFIX)',
             '--directory', '<(PRODUCT_DIR)/observatory/',
             '--command', 'deploy',
           ],
         }
       ],
     },
   ],
 }