This reverts commit 2627634b.

Currently every Linux runner has this code to allow relative resource paths; this moves it into the framework so that any embedder can get this behavior without that code needing to be in the template.

Rolls buildroot to pick up std::filesystem support in our libc++

Adds annotation `keepToString` to opt out.

* Canvas: Remove some unused virtuals, and prepare for signature change

* Better vertices fix

* Add empty placeholders for old signature

These should only be used on host binaries for more detailed crash reports.
Installing the handler on targets (iOS/Android) may cause use to break existing
crash reporting mechanisms users may have installed themselves in the process.

This should work on Darwin & Linux for now.

Doing something like int* a = nullptr; *a = 12; or abort or tripping an
assertion should print something the following before program termination. We
can tweak the report further if necessary.

```
[ERROR:flutter/fml/backtrace.cc(110)] Caught signal SIGSEGV during program execution.
Frame 0: 0x10658342c void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*)
Frame 1: 0x106555070 void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*)
Frame 2: 0x106554f81 testing::Test::Run()
Frame 3: 0x106555dc3 testing::TestInfo::Run()
Frame 4: 0x1065570a1 testing::TestSuite::Run()
Frame 5: 0x106562a55 testing::internal::UnitTestImpl::RunAllTests()
Frame 6: 0x10658c22c bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*)
Frame 7: 0x1065625c3 bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*)
Frame 8: 0x106562445 testing::UnitTest::Run()
Frame 9: 0x105c8dc33 RUN_ALL_TESTS()
Frame 10: 0x105c8dbe6 main
Frame 11: 0x7fff7c2dc3d5 start
```

Known issue: This routines that generate the stack trace are not signal safe.
But since we only use the same before the process is terminating, this ought to
be fine. I’ll work in a separate patch to convert all the internals to be signal
safe. In the meantime, this will help us better identify the causes of flakes on
our bots.

Fixes https://github.com/flutter/flutter/issues/50244

We no longer package AOT artifacts as discrete blobs. The portable ELF loader
from the testing library may be used instead.

Fixes https://github.com/flutter/flutter/issues/52263

Make the test harness reusable for other tests that want to launch a Dart VM

This reverts commit fdabcadd.

Also enable tests that were disabled due to MessageLoop flakiness resolved by https://github.com/flutter/engine/pull/16568

This change creates a test only implementation of flutter::Surface backed by an
offscreen Vulkan GrContext. Much of the code in this test Surface was lifted
from flutter::VulkanWindow which I was unable to use without extricating it
from the VkSurface/VkSwapchain code which we do not want to use in offscreen
tests. I would recommend refactoring VulkanWindow to separate GrContext
creation and VkSwapchain creation in order to promote greater code reuse
between onscreen and offscreen vulkan paths.

This change is excersised thoroughly by the shell tests and was manually
tested against these tests on Fuchsia on Intel.

* opt out dart:ui from nnbd

* Annotate tests as well

This was already enabled by-default in AOT mode in [0] - which made the
gen_snapshot invocations use "--lazy-async-stacks --no-causal-async-stacks".

See go/dart-10x-faster-async for more information.

[0] https://github.com/flutter/flutter/commit/347823234fd

This reverts commit 594f6608.

This change creates a test only implementation of flutter::Surface backed by an
offscreen Vulkan GrContext. Much of the code in this test Surface was lifted
from flutter::VulkanWindow which I was unable to use without extricating it
from the VkSurface/VkSwapchain code which we do not want to use in offscreen
tests. I would recommend refactoring VulkanWindow to separate GrContext
creation and VkSwapchain creation in order to promote greater code reuse
between onscreen and offscreen vulkan paths.

This change is excersised thoroughly by the shell tests and was manually
tested against these tests on Fuchsia on Intel.

This shouldn't result in any logical changes. I've done a quick smoke
test by building a local Android engine and running Flutter gallery, no
compile errors or other obvious issues.

Applied by running `/ci/format.sh | patch -p0` with the altered script
added in flutter/engine#16500. I did locally modify the script slightly
further so it would run against all Java files in the repo instead of
just modified ones.

This also adds a dependency on SwiftShader's Vulkan frontend.

No change in behavior expected. We have 2 frames in flight as before. By switching to Present2 and specifying a kMaxFramesInFlight however, we allow us greater flexibility to change how Flutter schedules its frames.

This change also adds tests for SessionConnection and VsyncRecorder.

This allows for targets to depend on the assets generated by the test_fixtures target.

Our tests depend on explicit latching to verify assertion are checked. If a test
does not respond for a long time, it has probably encoutered a deadlock. Instead
of waiting for the test runner to detect this, apply a very aggresive timeout on
a per test basis.

This was only necessary when the Engine had to build in multiple buildroots
where the sources where checked out at different paths relative to the
buildroot. This is no longer the case and there are already cases GN rules
have been written that mix and match variable usage with the direct
specification of the path to the Flutter sources relative to the sole buildroot.

Also update all known test harnesses to use this and fixes the broken shell_unittests harness.

Fixes https://github.com/flutter/flutter/issues/49853