This updates to mojo commit d259eb58aa59 and limits the roll script to
only pull in the parts of //mojo that are currently being used. More
stuff will be dropped in the future.

This means that //mojo/public is pulled in by gclient using the hash
specified in the DEPS file from the github.com/domokit/mojo_sdk mirror.
Each mojo_sdk commit corresponds to its originating
github.com/domokit/mojo commit and has the origin commit in the
message. Thus to find the mojo_sdk revision to use for a particular
mojo revision, grep through the commit log in the mojo_sdk repo for the
matching "Cr-Mirrored-Commit:" line.

If you need to back your local checkout to earlier than this commit
after running gclient, do 'rm -rf mojo/public && git reset --hard' to
get the locally checked in versions back.  Otherwise you'll get subtree
commits in mojo/public/... and have a bad time.

This reverts commit 2042a0f0.

Tracking issue https://github.com/domokit/sky_engine/issues/654

This patch makes the sky package match the structure we've adopted for the
other Dart packages we produce from this repo.

We now use `pub` to manage Dart packages.

Fixes #306

TBR=johnmccutchan@google.com

We no longer use sky_server.go. This patch removes it.

We now use the Dart sky_server in shelldb.

At gclient sync time, we now run `pub get` for every Dart package except
`workbench`. This patch ensures that we have all the packages we need locally.

R=chinmaygarde@google.com

After this patch, we use pub to manage versioning and fetching the material
design icons.

- angle
- boringssl
- smhasher
- cython

Remove these dependencies required stripping down some BUILD.gn files to remove
targets we don't build.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1245683003 .