Fixes #21113.

Underlying issue with breakpoint was that /var path is a symlink to a /private/var on mac. Because of that breakpoint could not be resolved.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

I'm working on fixing up all tests that rely on fs.currentDirectory to allow us to run concurrently. This one proved to be really tricky because it calls a lot of code and spawns multiple processes that have a mix of absolute and relative paths passed to them. I managed to handle some of it with a ChrootFileSystem, but the paths passed to the external processes were wrong.

The functionality here is covered by all of the integration tests that use flutter_tester that were added after this, including flutter_run_test.dart, lifetime_test.dart etc. in this same folder but because they're run through `flutter run` as a separate process they get to set their own working directories without affecting other tests if run concurrently.

* Add a detach command to detach without terminating (#21376)

* Add a detach command to detach without terminating

Fixes #21154.

* Bump protocol version for app.detach

* Tweak to detach/quit text

* Change logPrefix to named param

* Fix the text that the devicelab attach test looks for

* Rename test file

* Fix detection to work regardless of whether there are other devices

On Windows we get one message, but on Mac we get another (because of the Simulator always being available).

Revert "Allow FlutterTester to be provided with the working directory for execution (#21119)" (#21453)

This reverts commit 857bdc7a.

* Allow FlutterTester to be provided with the working directory for execution

Previously this test set fs.currentDirectory which prevents running tests concurrently. This allows setting the working directory for a FlutterTester in the cosntructor (optionally) and passes it through from the test (without setting fs.currentDirectory).

* Remove trailing whitespace

* Add a detach command to detach without terminating

Fixes #21154.

* Bump protocol version for app.detach

* Tweak to detach/quit text

* Change logPrefix to named param

* Add a failing test for #21418

* Filter workflows to only those applicable to the current platform

Fixes #21418.

* Unskip integration tests on Windows that now pass

The referenced issue is closed and these tests pass for me locally on Windows now.

* Remove import that's no longer used

* Add another issue that is afffecting these tests on Windows

* Add a hot restart test and mark skip on Windows

Skipped due to https://github.com/flutter/flutter/issues/21348.

* Pass uri instead of filepaths when hot-reloading.

Bug: https://github.com/flutter/flutter/issues/21168

* Remove imports. Enable another test

* Restore skipping of test on mac

This is a bit of a stab in the dark for a flake sometimes seen on bots (but I can't repro locally) #20822. Detaching from a Flutter app currently seems to terminate it, so it may be the cause of the "app not found" when trying to stop the app subsequently.

This change means we stop the app from the main process first, before terminating the attach process.

* Don't time out if process terminates when told to stop but response was not processed

While trying to reproduce #20822 I found a condition where the app may quit before the `app.stop` response is received but this code (which is called in integration test teardown) would sit around waiting and time out. With this change, the process exit is considered a valid response to the stop request.

* Add process exit to log for better errors in failures

This log can be turned on for debugging but is also dumped when a test fails (in certain conditions). With this in the log, it'll be clear if the timeout is because we're waiting for an event but the process quit.

* Unskip expression evaluation integration tests for non-Windows

* Add additional into to skip comments

* Add missing import

This test is sometimes flaky (#20822) because the two processes try to stop the same app. This fix changes to just gracefully terminate the attach process without explicitly trying to stop and then uses the original spawning process to stop the app.

I can't repro the flake locally to be certain, but I've verified only one stop command is sent now so it *should* be good.

Due to CPU contention we've seen these go really slow on Cirrus (see https://github.com/flutter/flutter/issues/19542#issuecomment-414265152), and there's also a chance our flakes are timeouts due to running slow rather than hanging.

* Don't keep parsing results once we've had the event we expected

* Report errors if an app.stop event is received while waiting for another event

* Don't throw if we see app.stop event when we've just sent an app.stop request

* Improve debug print to include --start-paused if being used

* Improve wrapping

All temporary directory start with `flutter_` and have their random component separated from the name by a period, as in `flutter_test_bundle.YFYQMY`.

I've tried to find some of the places where we didn't cleanly delete temporary directories, too. This greatly reduces, though it does not entirely eliminate, the directories we leave behind when running tests, especially `flutter_tools` tests.

While I was at it I standardized on `tempDir` as the variable name for temporary directories, since it was the most common, removing occurrences of `temp` and `tmp`, among others.

Also I factored out some common code that used to catch exceptions that happen on Windows, and made more places use that pattern.

* Reland "Roll engine to version b148e628ec86b3a9a0382e0bcfae73f0390a8232 (#20427)"

This is a re-land with downgraded `package:flutter_gallery_assets`
version.

* Downgrade package:flutter_gallery_assets to 0.1.4

* Change engine.version to 81baff97c29bb08cbf8453a3f9042c5813f84ad3 (which contains an additional fix)

* Change engine.version to e3687f70c7ece72000b32ee1b3c02755ba5361ac (since mac tarballs are corrupted on earlier commit)

Reason for revert: The package:flutter_gallery_assets has removed some images which are required for the examples/flutter_gallery, so the gallery build is failing (only discovered after landing, since gallery doesn't seem to get built during github PR presubmit checks)

This CL

  * rolls `engine.version` to flutter/engine@b148e628 (which includes dart sdk 2.1.0-dev)
  * rolls `goldens.version` to flutter/goldens@6c45fafdf (which includes updates due to skia changes in engine)
  * changes `platform.dill` to `platform_strong.dill` in various places due to flutter/engine@a84b210b
  * adds explicit `environment: sdk: ">=2.0.0-dev.68 < 3.0.0"` constraints to `pubspec.yaml` and `pubspec.yaml.tmpl` files (since pub defaults to `<2.0.0` if omitted) 
  * upgrades to newer versions of various 3rd party packages (to ensure transitive dependencies have `<3.0.0` sdk constraint)

* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test

* Skip tests instead of expecting failure, because expected failure = timeout which is slow

Some of these tests are known to fail and were coded to expect it; however because the failure is a timeout it's just dragging out test runs (shortening timeouts will make the tests flakier).

* Make it easier to find skipped tests

* Change SKIP to TODO

It was causing problems rolling Flutter into Fuchsia

* Skip integration tests that use flutter-tester on Windows

This is known to be broken. I will un-skip all of these once a fix for flutter-tester lands on Windows.

* Save with added import!

* Add tests to ensure flutter run does not quit at startup

These tests require https://github.com/dart-lang/vm_service_client/pull/38 to be merged and currently the third test (pause-on-exceptions) fails because of a bug.

* Remove TODO

Probably makes most sense this way for now.

* Address PR comments

* Look up breakpoints rather than relying on magic numbers

* Make breakpoint comment more obvious

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

* Extract some of startApp into a reusable method

* Get basic attach --machine working

* Attach --machine tweaks

Move validation to validate method and create daemon early so we get the startup event before trying to get a connection.

* Bump daemon version so we know whether it's valid to flutter attach

* Tweak output text

* Swap package imports for relative

* Review tweaks (naming, formatting, typedefs)

* Separate arguments from process spawning

This will make calling attach easier

* Add a basic test for flutter attach --machine

* Fix crash if port unforward modifies the list of forwarded ports

* Add a no-op port forwarder for flutter-tester

* Switch to using BasicProject instead of our own inline code

* Fix expectation in test now we have a portForwarder

* Remove stale TODO (this is done)

* Tweak formatting

* Change some Completers to void to fix Dart 2 issues

* Make cleanup of flutter processes in tests more reliable

* Fix quit signals

I confused SigInt&SigTerm for SigTerm&SigKill when I first did this. SigTerm can be blocked and doesn't guarantee the process will be terminated.

* Don't use deprecated constants

* Fix typo

* Add some additional info to debug buffer

* Fix return types on Futures