Add a script to sky pub package to download assets
Add a script to sky pub package to download assets

This is a fork of sky/tools/download_material_design_assets
which does not depend on depot_tools and assumes that its
right next to the assets directory instead of finding it
relative to the mojo root.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1106373002

flutter/flutter,tvolkert/flutter,tvolkert/flutter,jason-simmons/flutter,cbracken/flutter,jason-simmons/flutter,aghassemi/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,aghassemi/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,aghassemi/flutter,Hixie/flutter,jason-simmons/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,turnidge/flutter,turnidge/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter,Hixie/flutter,aghassemi/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,jason-simmons/flutter,Hixie/flutter,flutter/flutter