Enable the `unnecessary_new` lint to prevent regressions.

Enabled a bunch of lints which are (hopefully) not controversial and fix
existing violations. Most of the fixes are pretty mechanical.

Closes #1952

Towards #1952

Drops support for the flag and updates tests that were depending on it
or referring to it, but does not remove the supporting code. That will
be done in a followup.

- Parse but ignore the flag and warn when it is used.
- Drop tests specifically for the packages dir behavior.
- Remove unnecessary references to the flag.
- Drop utility test code and constants for the packages dir.
- Wherever possible replace checks for content within the packages dir
  with checks for content in the package spec, and sometimes cache.
- When working with git shas which are unpredictable and have an ugly
  format in the package spec check instead for lines that contain the
  sha, or which have a package spec which either specificially did or
  did not change after an upgrade.

This library only exists to be an "entrypoint" in `lib/` rather than
`lib/src/` and allow all other files to get rolled up into a single
module. We don't publish this package anyway, but an `import` is more in
line with our intentions here than `export`.

See discussion in https://github.com/dart-lang/pub/pull/1955

Fixes #1951

Continues to allow _creating_ packages directories for now, but if they
exist and aren't being created they won't be touched.

Testing from kernel files saves time over testing from source. This does
not switch Travis to test this way yet, but adds the deps to make it
easier to use locally.

Add a non-src lib file so get better behavior out of the coarse module
strategy. Without this file every library ends up becoming it's own
module.

In the simple error reporting example, root depends on baz ^1.0.0, not
baz ^3.0.0

* Look for the new snapshot error message from the Dart 2 VM.

* Reformat.

* 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The tests around outdated snapshots are failing because the old code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.

Call p.normalize() after p.absolute().

Fixes #1917

Add AnalysisContextManager and create an analysis context per package. Use public apis to parse to AST and find imports.

Closes https://github.com/dart-lang/sdk/issues/33256

The details of variable expansion when setting the PATH variable are a
little hard to follow. `"$HOME"` should work in more places than `"~"`

Generate Dart 2 snapshots when running in Dart 2 mode

This doesn't effect existing behavior, since any time snapshots are
allowed to be recompiled they've already been generated. However, it
will be important once we add support for Dart 2 snapshots, which will
may need to be recompiled when coming from Dart 1 mode.

All logic for when to run from snapshots is now shared.

Every entrypoint that isn't the one auto-created for the current
working directory is global.

See #1793 

The cache is derivable from the entrypoint.

* Use new dart:io constants
* Use latest args package
* use latest analyzer package

Follow-up to https://github.com/dart-lang/pub/commit/3d7ae0cdf0
Hopefully really fixes https://github.com/dart-lang/pub/issues/1887

Closes #1756

Also clean up to use Platform.isWindows everywhere

Closes #32188

Closes #1891

Restore the options that used to exist but mark them all as hidden.

This rewrites failures' incompatibility graphs so that they refer to
real version numbers rather than the pessimistic ranges that
pub_semver can now generate.

Closes #1819

Retry on pub server status 500

Fixes https://github.com/dart-lang/pub/issues/1887

Previously, pub retried up to 5 times when it received a response with
HTTP status 502, 503, 504. This adds 500 to the list of retryable
response statuses.