Add flutter tool support for creating app-specific VM snapshots.

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)

* 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.

Allows one to use a different core snapshot without a custom engine build by just packaging a different one in the APK.

We've always cached kernel compiles during bundle builds. This adds
caching for kernel compiles during AOT builds by moving caching into the
KernelCompiler.compile() method rather than around each invocation of it.

This also filters buildbot paths included by the kernel compile that are
not present on the local machine at paths /b/build/slave/Linux_Engine/...
The kernel compiler should probably include an option to not emit these
paths in the depfile, since these are used both by Gradle and the
Fingerprinter class.

Updates the message emitted when a kernel compile is skipped in the
build bundle action. Since we now use fingerprinting to enable
performance of script snapshots, AOT snapshots, and kernel compiles,
this helps a bit with debugging.

Also switches to trace level logging.

Adds a Fingerprinter utility class that can be used to compute unique
fingerprints for a set of input paths and build options, compare to the
output of a previous run, and skip the build action if no inputs or
options have changed. The existing Fingerprint class still does all the
heavy lifting. Fingerprinter adds common operations such as
reading/writing/comparing fingerprints and parsing depfiles.

This migrates existing uses of Fingerprint over to Fingerprinter.

This also adds better fingerprinting to AOT snapshotting, which
previously failed to include several options in its fingerprint
(--preview-dart-2, --prefer-shared-library).

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.

Wraps the compile function in a class injected via the global context,
which makes it easier to mock in unit tests -- specifically tests for
AOT snapshotting, which already require pretty significant amounts of
mock inputs.

* 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

* Introduce fingerprint check for preview-dart-2 build flx.

* Don't count outputs into fingerpint. Refactor compilation code.

* Revert "Revert "Reland pass filesystem options (#15622)" (#15651)"

This reverts commit 0f3eada0.

* Fix for windows failing test

* !isEmpty -> isNotEmpty

This reverts commit bbcf55a5 as it breaks hot reload on Windows.

* Revert "Revert "Pass filesystem-root, filesystem-scheme options through `flutter run, build` and gradle. (#15592)" (#15621)"

This reverts commit c663e336.

* Check whether filesystem-root is emtpy.

* Restore broken merge

* Fix windows test

* Adjust regex in test. Invalidate files by file path.

Revert "Pass filesystem-root, filesystem-scheme options through `flutter run, build` and gradle. (#15592)" (#15621)

This reverts commit 0f659fb1 as it
breaks bin/tasks/gradle_plugin_test.dart.

* Pass filesystem-root, filesystem-scheme options through flutter run/build/gradle.

* Use depfile in --preview-dart-2 mode.

* Generate and use frontend_server.d to guard against reusing outdated dill files when frontend_server changes.

* Put generated kernel files into <build> directory

* USe default value for kernel file

Pull code out of flx.dart:assemble() to make flx.dart:buildAssets
which creates the AssetBundle. This will allow us to create just
this instead of an entire FLX.

* Make the current command injected into the AppContext, allowing
  other classes to inject the current command.
* Introduce `AssetBundleFactory`, an injected factory class for
  spawning instances of `AssetBundle`. This allows other run contexts
  to use custom asset bundling logic.
* Clean up RunCommand by removing a 'packages' argument that duplicated
  a global argument by the same name (and for the same purpose).
  Duplicate arguments are confusing and error-prone.

Fixes https://github.com/flutter/flutter/issues/14058.

* Reland 9534082f with fix for incremental compilation.

When in incremental mode, awaiting exitCode won't work because compiler is not expected to exit after compilation.
Instead listen for stdout stream closing and report error if outputFilename has not been received.

* Fix lints

* Fix handling --preview-dart-2 for ios

* final var

This reverts commit 9534082f.

Causes hot_mode_dev_cycle__preview_dart_2_benchmark test timeout.

* Check frontend_server exit code.

When frontend_server completes abnormally, flutter tools has to stop and let user know.

* Add tests

* Plumb a --strong option through to the front end server and the engine
so that we can run flutter apps in preview-dart-2 and strong mode

* - Address analyzer lint issues
*- correctly set up strong mode option in the case of AOT builds

* Use IKG for restarts, use IKG with ProtectedFileByteStore.

* Fix comment, add end of file newline.

* Remove unused import

This adds --preview-dart-2 flag that enables use of Dart 2.0 Frontend in Flutter tools.

Extract a Snapshotter class that can be shared between FLX snapshotting,
AOT snapshotting, and assembly AOT snapshotting. Allows for better
testability of snapshotting logic.

* Extracts script snapshotting used in FLX build.
* Adds tests for snapshot checksumming, build invalidation/skipping.

Remaining work: disentangle + extract AOT snapshotting and Assembly AOT
snapshotting logic from build_aot.dart.

This change re-introduces skipping AOT snapshot builds if input sources
and outputs have not changed since the last snapshot build, assuming a
build for the same platform in the same build mode.

This reverts commit 3d5afb5a.
It includes the following changes relative to the original:
  1. Include the entrypoint source in the checksums
  2. include the build mode in the checksums
  3. include the target platform in the checksums

This change ensures that snapshot build checksums used to avoid
duplicate builds are invalidated by a change to framework revision
(in case gen_snapshot is updated), as well as by build mode.

Currently, only FLX snapshotting uses checksums to avoid duplicate
builds. FLX snapshotting is always done with BuildMode.debug, so didn't
include build mode in the checksum file.