* Move pub fully onto Dart 2 and off Dart 1.

- Have the tests run pub in Dart 2 mode. This unfortunately makes them
  much slower when running from source, but creating a pub snapshot
  helps.
- Only generate Dart 2 snapshots when precompiling.
- Remove tests around Dart 1 snapshots.
- Remove test utility code for running pub in Dart 1 mode.
- Change pub [global] run [un]checked mode tests to use assert().

The tests around outdated snapshots are failing because the old code
for detecting a bad snapshot version error does not seem to work in the
Dart 2 VM. I have an email thread going for that, but I wanted to send
this out in the meantime.

* Fix pub SDK snapshot extension.

* Fix pub now that the VM no longer supports "--preview-dart-2".

Pub fails to create snapshots on install because it's passing a flag
the VM no longer recognizes. This fixes that.

Also fixed a couple of runtime cast errors in a test.

* Create Dart 1 snapshot in Dart 1 mode.

This allows packages to declare features with dependencies, and to opt
in to those features in dependency declarations.

See #1593

This reverts commit 830e1481.

This wasn't intended to be committed yet.