Introduce dart_host_toolchain arg, remove '--checked' option from patch_sdk.py

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")
+import("../../build/dart_host_toolchain.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.
   #
@@ skipping 16 matching lines @@
 
 # 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 if (!prebuilt_dart_exe_works) {
-  pub_build_deps += [ "../bin:dart_bootstrap($host_toolchain)" ]
+  pub_build_deps += [ "../bin:dart_bootstrap($dart_host_toolchain)" ]
 
-  dart_out_dir =
-      get_label_info("../bin:dart_bootstrap($host_toolchain)", "root_out_dir")
+  dart_out_dir = get_label_info("../bin:dart_bootstrap($dart_host_toolchain)",
+                                "root_out_dir")
   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" ]
 }
@@ skipping 186 matching lines @@
 
 source_set("standalone_observatory_archive") {
   deps = [
     ":standalone_archive_observatory",
   ]
 
   sources = [
     rebase_path("$root_gen_dir/observatory/standalone_archive_observatory.cc"),
   ]
 }