Fix unnecessary Observatory rebuilds in Flutter

runtime/observatory/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Currently paths here are hard coded for convenience in building Mojo/Flutter.
 declare_args() {
   # Specify the path to a host compatible version of the Dart SDK.
   # This SDK is used to compile the Observatory frontend sources.
   dart_host_sdk = rebase_path("//third_party/dart-sdk/dart-sdk")
 
@@ skipping 59 matching lines @@
 
   inputs = [
     script,
     "$root_gen_dir/observatory_copy/dart/runtime/observatory/pubspec.yaml",
   ]
 
   args = [
     "--silent=True",
     "--pub-executable",
     dart_host_pub_exe,
+    "--stamp",
+    rebase_path("$root_gen_dir/observatory_copy/dart/runtime/observatory/packages.stamp"),
     "--directory",
     rebase_path("$root_gen_dir/observatory_copy/dart/runtime/observatory/"),
     "--command",
     "get",
   ]
 
   outputs = [
-    "$root_gen_dir/observatory_copy/dart/runtime/observatory/pubspec.lock",
+    "$root_gen_dir/observatory_copy/dart/runtime/observatory/packages.stamp",
   ]
 }
 
 action("pub_build_observatory") {
   sources =
       rebase_path(observatory_sources_gypi.sources,
                   "",
                   "$root_gen_dir/observatory_copy/dart/runtime/observatory")
 
   deps = [
     ":copy_observatory",
     ":copy_observatory_deps",
   ]
 
   script = "//dart/tools/observatory_tool.py"
 
   inputs = [
     script,
+    "$root_gen_dir/observatory_copy/dart/runtime/observatory/packages.stamp",
   ]
   inputs += sources
 
   args = [
     "--silent=True",
     "--pub-executable",
     dart_host_pub_exe,
     "--directory",
     rebase_path("$root_gen_dir/observatory_copy/dart/runtime/observatory/"),
     "--command",
     "build",
     rebase_path("$root_out_dir/observatory/build"),
   ]
 
   outputs = [
-    "$root_out_dir/observatory/build/web/index.html",
     "$root_out_dir/observatory/build/web/index.html.polymer.bootstrap.dart.js",
   ]
 }
 
 action("deploy_observatory") {
   deps = [
     ":pub_build_observatory",
   ]
 
   script = "//dart/tools/observatory_tool.py"
 
   inputs = [
     script,
-    "$root_out_dir/observatory/build/web/index.html",
     "$root_out_dir/observatory/build/web/index.html.polymer.bootstrap.dart.js",
   ]
 
   args = [
     "--silent=True",
     "--pub-executable",
     dart_host_pub_exe,
     "--directory",
     rebase_path("$root_out_dir/observatory"),
     "--command",
     "deploy",
   ]
 
   outputs = [
-    "$root_out_dir/observatory/deployed/web/index.html",
     "$root_out_dir/observatory/deployed/web/index.html.polymer.bootstrap.dart.js",
   ]
 }
 
 action("archive_observatory") {
   deps = [
     ":deploy_observatory",
   ]
 
   script = "//dart/runtime/tools/create_archive.py"
 
   inputs = [
     script,
-    "$root_out_dir/observatory/deployed/web/index.html",
     "$root_out_dir/observatory/deployed/web/index.html.polymer.bootstrap.dart.js",
   ]
 
   args = [
     "--output",
     rebase_path("$root_gen_dir/observatory/observatory_archive.cc"),
     "--tar_output",
     rebase_path("$root_gen_dir/observatory/observatory_archive.tar"),
     "--outer_namespace", "dart",
     "--inner_namespace", "observatory",
     "--name", "observatory_assets_archive",
     "--client_root", rebase_path("$root_out_dir/observatory/deployed/web/"),
   ]
 
   outputs = [
     "$root_gen_dir/observatory/observatory_archive.cc",
     "$root_gen_dir/observatory/observatory_archive.tar",
   ]
 }
 
 source_set("embedded_observatory_archive") {
   deps = [
     ":archive_observatory",
   ]
 
   sources = [
     rebase_path("$root_gen_dir/observatory/observatory_archive.cc"),
   ]
 }