Upload/Download dart_snapshotter

mojo/public/mojo.gni

 # Copyright 2014 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/module_args/mojo.gni")
 
 # If using the prebuilt shell, gate its usage by the platforms for which it is
 # published.
 mojo_use_prebuilt_mojo_shell = false
 if (!defined(mojo_build_mojo_shell_from_source) ||
     !mojo_build_mojo_shell_from_source) {
   mojo_use_prebuilt_mojo_shell = is_linux || is_android
 }
 
+# If using the prebuilt dart_snapshotter, gate its usage by the platforms for
+# which it is published.
+mojo_use_prebuilt_dart_snapshotter = false
+if (!defined(mojo_build_dart_snapshotter_from_source) ||
+    !mojo_build_dart_snapshotter_from_source) {
+  mojo_use_prebuilt_dart_snapshotter = host_os == "linux" || host_os == "mac"

[jamesr, 2015/08/17 22:10:16]

what is a hypothetical developer on a different host_os supposed to do?  it
doesn't seem like they have any real choices - even if our prebuilts don't work
they certainly won't be able to build one themselves (i.e. the label won't
exist). i think what would make more sense here is to assert that the
module_args either set use_prebuilt_dart_snapshotter or specify a label for the
snapshot binary and not have branches on host_os

[zra, 2015/08/17 22:51:27]

Added assert that dart_snapshotter_bin is defined when we decide that
mojo_use_prebuilt_dart_snapshotter is false, which is when
mojo_build_dart_snapshotter_from_source is undefined or true.

+}
+
 # If using the prebuilt network service, gate its usage by the platforms for
 # which it is published.
 mojo_use_prebuilt_network_service = false
 if (!defined(mojo_build_network_service_from_source) ||
     !mojo_build_network_service_from_source) {
   mojo_use_prebuilt_network_service = is_linux || is_android
 }
 
 # Enable Dart apptest framework by default.
 mojo_use_dart_apptest_framework = true
@@ skipping 13 matching lines @@
 mojo_use_network_in_sdk = true
 if (defined(mojo_disable_network_in_sdk) && mojo_disable_network_in_sdk) {
   mojo_use_network_in_sdk = false
 }
 
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location
 # of the public SDK within a client repo.
 mojo_root = get_path_info("../..", "abspath")