Pub: Roll pub into dart-sdk

[jonasfj]

We've recently added dependency on package:tar to avoid using system tar implementation.
This means migrating into Dart SDK will involve a bit of moving around with dependencies.

Specifically:

• [x] Land ChunkedStreamReader in package:async (https://github.com/dart-lang/async/pull/161)
• [x] Migrate package:tar to use ChunkedStreamReader from package:async
• [x] Vendor https://github.com/simolus3/tar/ in pub
• [x] Re-land pub in Dart SDK (sdk/+/192304)
• [x] Stop packaging 7zip with Dart SDK on Windows.

[simolus3]

I've just published (and tagged) version 0.3.3 of package:tar. It only depends on the async, meta and typed_data packages, which are available in the SDK repo.

[jonasfj]

Filed https://dart-review.googlesource.com/c/sdk/+/199040

[jonasfj]

https://dart-review.googlesource.com/c/sdk/+/199250

[jonasfj]

Starting 2.14 Dart should no longer include 7zip on windows, and not system tar on linux :D

With a little luck it'll be in the first 2.14 beta :rocket: