Make :embedded_observatory_archive compilable on Win for 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.
 
 import("../../build/executable_suffix.gni")
 import("../../build/prebuilt_dart_sdk.gni")
 
 # 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")
 
   # Specify the path to a host compatible version of pub.
   # This is used to compile the Observatory frontend sources.
   #
   # The observatory build tool searches for pub first using this GN arg, then
   # for the checked-in SDK, and if that doesn't work, it uses the dart_bootstrap
   # built as part of the build to invoke pub. If this arg is set to a non-empty
   # string, we assume that there is no need for dart_bootstrap. If it is set to
   # "", then we assume that there may be a need for dart_bootstrap.
-  dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub")
+  if (defined(is_win) && is_win) {
+    dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub.bat")
+  } else {
+    dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub")
+  }
 }
 
 # We set this to "" in the Fuchsia build to force building with dart_bootstrap.
 if (defined(is_fuchsia) && defined(is_fuchsia_host) &&
     (is_fuchsia || is_fuchsia_host)) {
   dart_host_pub_exe = ""
 }
 
 # Construct arguments to the observatory tool for finding pub.
 pub_build_deps = []
@@ skipping 207 matching lines @@
 
 source_set("standalone_observatory_archive") {
   deps = [
     ":standalone_archive_observatory",
   ]
 
   sources = [
     rebase_path("$root_gen_dir/observatory/standalone_archive_observatory.cc"),
   ]
 }