GN: Don't depend on dart_bootstrap if there is a usable prebuilt sdk

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("../../utils/invoke_dart.gni")
+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")
 }
 
 # Construct arguments to the observatory tool for finding pub.
 pub_build_deps = []
 pub_build_args = []
 if (dart_host_pub_exe != "") {
   pub_build_args = [
     "--pub-executable",
     dart_host_pub_exe,
   ]
-} else {
+} else if (!prebuilt_dart_exe_works) {
   pub_build_deps += [ "../bin:dart_bootstrap($host_toolchain)" ]
 
   dart_out_dir =
       get_label_info("../bin:dart_bootstrap($host_toolchain)", "root_out_dir")
-  dart_bootstrap =
-      rebase_path("$dart_out_dir/dart_bootstrap$dart_executable_suffix")
+  dart_bootstrap = rebase_path("$dart_out_dir/dart_bootstrap$executable_suffix")
 
   pub_build_args = [
     "--sdk=True",
     "--dart-executable",
     dart_bootstrap,
   ]
+} else {
+  pub_build_args = [ "--sdk=True" ]
 }
 
 current_dir = rebase_path(".", "//")
 
 # Helper build rules for packaging the Dart observatory resources.
 observatory_sources_gypi =
     exec_script("../../tools/gypi_to_gn.py",
                 [ rebase_path("../observatory/observatory_sources.gypi") ],
                 "scope",
                 [ "../observatory/observatory_sources.gypi" ])
@@ skipping 177 matching lines @@
 
 source_set("standalone_observatory_archive") {
   deps = [
     ":standalone_archive_observatory",
   ]
 
   sources = [
     rebase_path("$root_gen_dir/observatory/standalone_archive_observatory.cc"),
   ]
 }