See #18879.

This reverts commit 5d8ea59d.

* Improve update checking

This change emables pinging the server to check for updates regardless of whether the local version is "out of date". The server code already has a 7-day cache so the result is that we can now ping the server once every 7 days instead of waiting for the local install to be 4 weeks out of date before pinging.

The original 4 week period is still used for when we'll start warning the user they're out of date if we could not confirm with the server whether there's a new version.

* Improve message when we know there's a new version available

* Fix bnullable bool checks

* Switch nullable bool to enum

* Fix casing of enum values

* Remove stale comment

The names are now descriptive so doesn't need additional explanation.

* Improve name of function

* Remove note:

* Rename kPauseToLetUserReadTheMessage -> timeToPauseToLetUserReadTheMessage

* Change kVersionAgeConsideredUpToDate to 5 weeks from 4

* Inline the isNewerFrameworkVersionAvailable check

* Fix indenting (?)

* Fix more indenting

* Rename function to be clearer it's getting the commit date

* Formating tweaks

* Update stamp when connection failed, and reduce time before we'll try again

Previously we would hit the server on every command if we thought we might be out of date and we never successfully connected (eg. if you're offline). This makes the stamp update even when there's a conneciton failure so that this won't happen, but reduces the time till we check again from 7 days to 3 days to compensate a little in case it was a one-off.

https://github.com/flutter/flutter/pull/18193#issuecomment-399222269

* Fix comment

* Don't perform update checks if not on an official channel

This test is failing on mac_bot (but passing elsewhere) because flutter-tester is apparently quitting earlier than expected. Locally it fails with an even weirder error and almost all tests are failing with "Compilation failed" (this isn't happening on the builds, so something is bad on my MacBook). Marking as skip to fix build while investigating; there's no real impact of this test not running; it's testing a tool that its itself used for testing (and not currently in any way that should be affected by this failure).

Fixes https://github.com/flutter/flutter/issues/18200.

This should fix https://github.com/flutter/flutter/issues/18729

I'll add a gold test, and try to figure out how to remove the saveLayer without affecting the correctness later.

This should fix https://github.com/flutter/flutter/issues/18731 and I've added a unit test for it.

Introduced in d248725e, but has been red
since landing. Marking flaky rather than reverting since it's the only
failure.

The `flutter attach` test was failing because the Android device had
the app already installed.

Add `flutter attach` command.

This command allows Flutter to connect to applications that haven't been launched using `flutter run`.

Unbreaks flutter-build

* Revert changes to core SDK members that haven't been propagated to the g3 Dart SDK yet.

* Respond to comments.

* Remove retype

* Temporarily un-remove VMService.retype to unblock g3 roll

* Checkout back to master, we need fewer changes to reach compat with dev.61 than dev.60

This prevents the error:

`type 'List<dynamic>' is not a subtype of type 'List<Map<String, dynamic>>' `

The test added fails when dart_vm.dart is reverted to `master`, and succeeds in
its current state.  This issue was run into while connecting to the
Dart VM.

This fixes https://github.com/flutter/flutter/issues/18708

* Suppress stdout compile diagnostic when compile expressions.

This fixes https://github.com/flutter/flutter/issues/18574.

* Use equal sign for default values