This updates AppContext per the recommendations in #15352

Fixes #15352

* Pass entry points JSON files to front-end server (#15180)

* Fix ios/debug build which uses 'flutter build aot': do not require entry points files and disable AOT transformations in this mode

* Revert "Pass entry points JSON files to front-end server (#15180)"

This reverts commit 0edea887.

Reason: buildbot failures on Mac.

* Add to Artifacts, and add optional arguments to entry points for flut… (#15185)

Add to Artifacts, and add optional arguments to entry points for flutter run and test to allow for wiring up the same with preview-dart-2 internally

* Changing packages uri here is not necessary

This reverts commit e0f6e626.

Add to Artifacts, and add optional arguments to entry points for flutter run and test to allow for wiring up the same with preview-dart-2 internally

This is needed to support running with local engine's dart sdk.

This is follow-up to https://github.com/flutter/flutter/pull/14702.

* Use already downloaded dart sdk for frontend_server.

This is follow-up to https://github.com/flutter/flutter/pull/14610

* Remove reference to engine dart-sdk from artifacts

* Remove unused imports

* WIP to switch back to solution where dart sdk is downloaded into bin/cache/dart-sdk, but it is 'flutter precache -a' that can download all dart sdks for other platforms under bin/cache/artifacts/engine/<platform>/dart-sdk

* Download all dart sdks when 'flutter precache -a'

* 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

Executables have '.exe' extension on Windows. We do have to specify extension for gen_snapshot since when running with local engine, we are looking for that exact file before launching it.

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

The output location of gen_snapshot differs based on the engine's target
platform, and we don't know the target platform when building a
platform-independent FLX in JIT mode.

Update --local-engine to account for changed host architecture of 32-bit Android debug builds. (#11263)

Roll engine to 03e642bc8940441bbad5c2fb7d3ca868c1fdad54.

Revert "Update --local-engine to account for changed host architecture of 32-bit Android debug builds. (#11251)" (#11259)

This reverts commit 3f514fcf.

Update --local-engine to account for changed host architecture of 32-bit Android debug builds. (#11251)

Roll engine to 7480e32854606e554ae37f8bd241c050bdc565c4.

* Remove legacy .apk build.

Print out an error message telling the user to upgrade the project if
it's not Gradle-based. Removed all the obvious traces of the legacy
build.

Added support for Dart VM kernel snapshots in Gradle builds.

Fixed Android installs to verify that the app is actually installed, and
not just rely on the presence of the .sha1 file.

gen_snapshot is now used for both JIT and AOT snapshots.  The JIT version
used in debug mode will be a 64-bit binary.

* eliminate sky snapshot

* roll engine

* Enable Hot Reload on Windows (backed by gen_snapshot)

\o/

Two caveats:
* Hot Reload on Windows is slower than on other platforms because gen_snapshot is slower then sky_snapshot
* We currently cannot hot reload projects with spaces in the path

* enable tests

Artifacts are now located in a central place.
This will enable us to downlaod artifacts when we need them (instead of
downloading them all upfront).
This also makes replacing sky_snapshot with gen_snapshot easier.

Artifacts from local engine builds will be found based on the --local-engine flag

* use the new cache code to locate more artifacts

* add todo

* review comments

* better messaging about windows support

* fix lints