This change adds Dart VM event support (listening for when a VM starts/stops by using a periodic heartbeat).

This also adds support to connect to a specific `IsolateRef` through the flutter driver, so that when an application spawns, it can immediately be driven (as shown in included example code).

The "about" text was apparently too long.

* Renamed CI tasks

`tests` to `tests-linux`
`windows` to `tests-windows`

* Explicitly specify shards everywhere

* Update issue templates

* Rename Custom.md to Z.md

* Create A.md

* Rename A.md to BUG.md

* Rename Z.md to SUPPORT.md

* Create ACTIVATION.md

* Update BUG.md

* Update BUG.md

* Delete ISSUE_TEMPLATE.md

In order to avoid checking binaries into flutter/flutter,
this comparator can be used to retrieve golden files from
a sibling flutter/goldens repository.

https://github.com/flutter/flutter/issues/16859

Changes since last roll:
```
ae0d3a553 Roll dart to 2765fcf2aecd3841d082fedaeafc00a73a965f8c. (#5160)
73b835c33 Roll src/third_party/skia/ 0caef298c..20fa1e94a (1 commit) (#5159)
f5c7cb366 Roll dart to b298fc6d8f6a0e1aa841dbbdda26663d6012a79a. (#5155)
93b85ee68 Add Android accessibility bridge support for "longPress" and "tap" semantic events
862d13c6f Roll src/third_party/skia/ 97b48a9c7..0caef298c (6 commits) (#5158)
9b7cc44df Dart bindings own the FDIO namespace. Release the engine side reference to the handle after bindings are setup. (#5156)
8896bd35a Roll src/third_party/skia/ f9ab22385..97b48a9c7 (11 commits; 1 trivial rolls) (#5154)
77910baad Roll src/third_party/skia/ 2050565fe..f9ab22385 (11 commits) (#5151)
0667cba31 Roll src/third_party/skia/ 3fef39d94..2050565fe (2 commits) (#5150)
5da171634 Roll src/third_party/skia/ ef4142a9b..3fef39d94 (1 commit) (#5148)
4ec4b3870 Roll src/third_party/skia/ c86c5c014..ef4142a9b (2 commits) (#5147)
d5c111717 Roll dart to 011676641a8b4b77bb372384c712709cbf037675 (#5146)
beb988c60 Roll src/third_party/skia/ 91368c9b9..c86c5c014 (4 commits) (#5145)
a58eabf14 Roll src/third_party/skia/ dc3192b30..91368c9b9 (12 commits; 1 trivial rolls) (#5144)
```

Currently, `flutter update-packages --force-upgrade` will
crash if it encounters a pubspec.yaml file with no dependencies
(either regular or dev). The assumption that we'd never see
such pubspec files is no longer valid, as we have such a file
in one of our tests.

Correctly split ARCHS into a Bash array in xcode_backend.sh.

Previously, when ARCHS contained multiple values (e.g., ARCHS="armv7 arm64"),
we treated that as a single architecture, and lipo invocation would
fail.

This contains the following commits:

flutter/engine@2812ea3 Define Uri.base as CWD to match the standalone Dart VM.
flutter/engine@6f15a91 Roll src/third_party/skia/ 4912d903b..dc3192b30 (7 commits)
flutter/engine@f9165be Roll src/third_party/skia/ c353ee211..4912d903b (1 commit)
flutter/engine@fb51bf2 Roll src/third_party/skia/ d5750b6b3..c353ee211 (1 commit)
flutter/engine@a859432 Roll buildroot to a69ebc4e1.
flutter/engine@89a8dff Roll src/third_party/skia/ 0ce19fa0b..d5750b6b3 (3 commits; 1 trivial rolls)

This reverts commit 2849bc04.

The engine roll causes hot reload crashes.

ProcessResult.stdout has static type dynamic so for
inference to infer proper type argument for the map
invocation we need to cast stdout to String explicitly.

Fixes #17163

d5c111717 (HEAD -> master, upstream/master) Roll dart to 011676641a8b4b77bb372384c712709cbf037675 (#5146)
beb988c60 (dart-roll) Roll src/third_party/skia/ 91368c9b9..c86c5c014 (4 commits) (#5145)
a58eabf14 Roll src/third_party/skia/ dc3192b30..91368c9b9 (12 commits; 1 trivial rolls) (#5144)
2812ea3ed Define Uri.base as CWD to match the standalone Dart VM. (#5137)
6f15a915c Roll src/third_party/skia/ 4912d903b..dc3192b30 (7 commits) (#5142)
f9165befd Roll src/third_party/skia/ c353ee211..4912d903b (1 commit) (#5141)
fb51bf280 Roll src/third_party/skia/ d5750b6b3..c353ee211 (1 commit) (#5140)
a8594324d Roll buildroot to a69ebc4e1. (#5139)
89a8dffcb Roll src/third_party/skia/ 0ce19fa0b..d5750b6b3 (3 commits; 1 trivial rolls) (#5138)

This updates the Android build to declare the kernel compile depfile as
an output and its contents as inputs when running with --preview-dart-2
(the default mode).

The 'flutter build aot' command behaves differently depending on whether
it's running in Dart 1 or Dart 2 mode:

* Dart 1: the entrypoint Dart file (typically main.dart) is passed
  directly to gen_snapshot, which then emits snapshot.d, whose contents
  list the transitive closure of Dart dependencies (input files) for the
  snapshot. snapshot.d is a declared output, its contents (plus
  gen_snapshot itself) constitute the set of input files to the Gradle
  build action.

* Dart 2: then entrypoint Dart file (typically main.dart) is first
  compiled with the Dart kernel frontend. This emits kernel_compile.d,
  whose contents list the transitive closure of Dart dependencies (input
  files) for the kernel 'dill' output file. This 'dill' file is then
  passed to gen_snapshot, which emits snapshot.d, whose contents are
  empty. As of this change, both snapshot.d and kernel_compile.d are
  declared outputs, and their contents (plus gen_snapshot and the
  frontend compiler themselves) constitute the set of input files to the
  Gradle build action.

This fixes a bug wherein profile/release AOT outputs were not
invalidated due to snapshot.d being empty, and kernel_compile.d being
ignored. This was introduced during recent refactoring of the AOT build
code, wherein the kernel compile and gen_snapshot actions were changed
to emit independent depfiles (previously one stomped -- or failed to --
on the other's output).

* Configure Cirrus CI

* Read file directly instead of shelling a command

* Wait for all streams to finish

* Add LTR `textDirection`

Since `RenderFlex#_debugHasNecessaryDirections` requires it when `direction` is horizontal and there are multiple children.

Previously, we were incorrectly passing --vm_snapshot_data and
--isolate_snapshot_data options to gen_snapshot in assembly AOT builds.
These only make sense in AOT blob snapshot mode (alongside
--vm_snapshot_instructions and --isolate_snapshot_instructions).

There's very little code-sharing between the two, and what little there
is is concentrated in the GenSnapshotClass and the fingerprint
reading/writing utility methods.

This enables support for a `flutter_test_config.dart` configuration file,
which will be discovered and handed the responsibility of running the
test file (thus allowing it to run pre-test setup on a project level).

https://github.com/flutter/flutter/issues/16859

This de-duplicates assembly AOT configuration between Android and iOS,
and makes it easier to adjust parameters for 32-bit iOS (which, like
32-bit Android, requires --no-integer-division) in an upcoming patch.

This is now derived automatically from the target platform by
gen_snapshot.

iOS debug builds always run in interpreted mode whether on device or on
simulator. In both cases, we can skip snapshotting and link against an
empty App.framework. Previously, we did this for iOS simulator builds.
This does the same for device builds.

Previously, debug iOS builds used gen_snapshot to generate a core
snapshot, then used 'xxd' to generate C files containing the snapshot
data in buffers named kDartVmSnapshotData and kDartIsolateSnapshotData,
which are then compiled/linked into App.framework. This is unnecessary
since the VM compiled into Flutter.framework already contains this data.

Bugfix: Moves AOT snapshot input verification past where the last input
is added to the inputs list.

Cleanup:
* Extracts _isValidAotPlatform method.
* Moves non-platform-specific logic to the top.
* Moves variable declaration closer to first use, and inlines to a
  narrower scope where possible.

This relands #17136, which was reverted in #17142 due to breakage in
on-device iOS debug builds.