* Fix coverage collection crash

Based on Jason's patch in https://github.com/flutter/flutter/pull/19546/

This is more or less the same but I tried to avoid using `dynamic`.

* Improve argument and variable names in flutter_platform

* Don't bother with reduce, since the order is guaranteed.

Turns out we weren't deleting the dill directory or the fonts directory.

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.

* Try to resolve an intermitted crash during coverage collection

The only theory I can come up with is that maybe the test completes
before we finish processing the standard input, so I made the test
harness wait for the observatory URL before considering whether the
test has finished or not.

Also, some code cleanup while I'm at it, e.g. avoiding using "onFoo"
for the names of methods, avoiding back-to-back switch statements with
the same values, avoiding `_` argument names, and using `?.` instead
of `if (foo != null) foo.`.

* Revert back the signature of _pipeStandardStreamsToConsole

* Also remove the other additions to this method.

* 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)

In certain cases, the test would fail before creating the (lazily created) compiler object, and then we'd
try to call shutdown() on null in those cases.

Fixes #18610

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

* Remove uses of deprecated constants
* Change int.parse to int.tryParse where appropriate

This reverts commit cc1cf13e.

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

* It's time to #deleteDart1 (#18293)

Eliminates support for Dart 1 in flutter_tools, and drops our Dart 1
benchmarks. All commands now run in Dart 1 mode only.

Eliminates --preview-dart-2 / --no-preview-dart-2 support.

* Fix indentation, remove no longer necessary .toList()

* Only push udpated kernel if >0 invalidated srcs

Several benchmarks started failing during kernel compile:
1. Timeouts:
   * commands_test
   * hot_mode_dev_cycle__benchmark
   * run_machine_concurrent_hot_reload
   * service_extensions_test

2. Kernel compiler crash:
   * hot_mode_dev_cycle_linux__benchmark
   * routing_test

3. Unexpected termination:
   * hot_mode_dev_cycle_win__benchmark

This reverts commit ed63e708.

Eliminates support for Dart 1 in flutter_tools, and drops our Dart 1
benchmarks. All commands now run in Dart 1 mode only.

Eliminates --preview-dart-2 / --no-preview-dart-2 support.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

This will allow external tools that wrap our test harness to share the
code that generates the test bootstrap.

This change exposed an issue whereby the LocalGoldenFileComparator
was being too strict in its URI handling, so this changes relaxes
that constraint as well (and adds associated tests).

The new values are not ready for use yet inside Google

* Roll engine to e976be13c51448f89107d082ec81e2b6731671fa

* move away from deprecated constants

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

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

* Add a `matchesGoldenFile()` async matcher that will match
  a finder's widget's rasterized image against a golden file.
* Add support for pluggable image comparison backends
* Add a default backend that does simplistic PNG byte
  comparison on locally stored golden files.
* Add support for `flutter test --update-goldens`, which will
  treat the rasterized image bytes produced during the test
  as the new golden bytes and update the golden file accordingly

Still TODO:

* Add support for the `flutter_test_config.dart` test config hook
* Utilize `flutter_test_config.dart` in `packages/flutter/test`
  to install a backend that retrieves golden files from a dedicated
  `flutter/goldens` repo

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

Fixes https://github.com/flutter/flutter/issues/16496

This avoids notifying the coverage collector that we
completed the test when in fact we timed out, which in
turn avoids a downstream exception caused by the fact
that coverage collector assumes it has a valid observatory
port by which it can gather coverage data.

Fixes https://github.com/flutter/flutter/issues/16839

Finalizers handle the restoration of state. In order to restore
the state correctly, they shouyld be run in LIFO order.

Fixes #16657

* Roll engine to d6d4eec24c38bb06a9b1aef4e70c321d447ba762

Contains the following engine commits:

flutter/engine@1bc0e1b On windows, refer to Dart snapshot directly executable.
flutter/engine@cf6ca32 Revert "Roll Dart to fe606f890b0a311da802c78b0af414a3c2087a79"
flutter/engine@4b45a53 Only make ERROR and FATAL log levels visible by default.
flutter/engine@98f6c2d Fix shell launcher test (dart2 compliant and stop polling)
flutter/engine@75851f0 Roll Dart to fe606f890b0a311da802c78b0af414a3c2087a79
flutter/engine@9495a52 On iOS, try to use ES3, then fall back to ES2.
flutter/engine@d6d4eec Fix broken Linux build
flutter/engine@d15dc76 Fix broken Chromebot build
flutter/engine@c17a629 Breadcrumbs to the wiki
flutter/engine@4eaf2c2 Return raw (unencoded) bytes in Image.toByteData()
flutter/engine@d812a61 Roll src/third_party/skia/ e669bdfad..276886160 (9 commits)
flutter/engine@62aeab7 Call SkSurface::flush instead of getting a backend handle in vulkan_swapchain.
flutter/engine@f4ebb18 Roll src/third_party/skia/ 3462eb034..e669bdfad (54 commits)
flutter/engine@0b7d6be Re-land "do not pause rendering when android view loses focus"
flutter/engine@91dee59 If the rasterizer has a valid surface context, use that to create a snapshotting render target.
flutter/engine@570231b Specify the packages file path when running engine dart tests.
flutter/engine@0b9cef5 Minor update to README
flutter/engine@c386418 Promote line height in text style created from paragraph style
flutter/engine@4e0fbb6 If the test specifies a .dill file, dont make the engine interpret is as source.
flutter/engine@58e84c8 Re-land "Support multiple shells in a single process.
flutter/engine@37e5df0 Remove only use of SkPaint::kGenA8FromLCD
flutter/engine@b7358b3 libtxt: cache font families that are remapped to the default font family
flutter/engine@a530035 libtxt: fix leaks in Skia object reference counting
flutter/engine@0c74fc9 [fuchsia] Fix use-after-free
flutter/engine@b6d2dde [fuchsia] Fix build
flutter/engine@1dd1264 [async] Adapt to libasync API changes.

* Run test platform finalizers in reverse order to ensure that shell is shutdown before trying to delete any temporary files or directories. This fixes "Access Denied" error on Windows (see #16657).

* Handle error count reported by frontend.

Extend compilation result from single string to a structure(string filename and integer error count).

* Use ?.

* Include engine roll with dart sdk roll.

* parse(onError) -> tryParse

* Make '?? throw' more readable and avoid issue with analyzer

* Fix test so it mocks compiler output including errors count

* Report an error if compilation times out instead of waiting forever.

* Remove braces