Upload/Download dart_snapshotter

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
index 2e2581400188d3a02c78d4c8249869244ae03e84..f32383e7035ab5897910a6037a0a99d67e9a48c9 100644
--- a/mojo/public/mojo.gni
+++ b/mojo/public/mojo.gni
@@ -12,6 +12,14 @@ if (!defined(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"
+}
+
 # If using the prebuilt network service, gate its usage by the platforms for
 # which it is published.
 mojo_use_prebuilt_network_service = false