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

build/dart_host_toolchain.gni

Index: build/dart_host_toolchain.gni
diff --git a/build/dart_host_toolchain.gni b/build/dart_host_toolchain.gni
new file mode 100644
index 0000000000000000000000000000000000000000..1813cc658e0f9474365ac1830717e84482da1b0f
--- /dev/null
+++ b/build/dart_host_toolchain.gni
@@ -0,0 +1,7 @@
+# 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.
+
+declare_args() {
+  dart_host_toolchain = host_toolchain

[zra, 2017/06/22 17:16:39]

Please add comments here explaining what this option does. In particular, if
dart_host_toolchain should not replace all uses of host_toolchain, this comment
should explain when to use one and not the other.

+}