Also makes the `pub` shell script (and its .bat counterpart)
forward to a `dart __deprecated_pub` command that implements the
old top-level.
This allows us to get rid of pub.dart.snapshot saving ~15 MB in a
unzipped sdk.
The reason for not forwarding directly to `dart pub` is that the
interface is slightly different (for example there is no `dart pub --version`).
The only new commit in pub is: `3c2ce330 Expose toplevel as a command`
Bug: https://github.com/dart-lang/pub/issues/2736
Change-Id: I6eb08d120c4844b3a12bc29544df6a868cd6fcc8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210582
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Jonas Jensen <jonasfj@google.com>