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

utils/kernel-service/BUILD.gn

 # Copyright (c) 2017, 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.
 
 import("../application_snapshot.gni")
+import("../../build/dart_host_toolchain.gni")
 
 application_snapshot("kernel-service") {
   dfe_script = "kernel-service.dart"
   deps = [
-    "../../runtime/vm:patched_sdk($host_toolchain)",
+    "../../runtime/vm:patched_sdk($dart_host_toolchain)",
   ]
   main_dart = dfe_script
   training_args = [
     "--train",
     "file://" + rebase_path("../../pkg/compiler/lib/src/dart2js.dart"),
   ]
 }