The new values are not ready for use yet inside Google

* Roll engine to e976be13c51448f89107d082ec81e2b6731671fa

* move away from deprecated constants

* 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'

Fixes https://github.com/flutter/flutter/issues/14524

* Revert "Revert "Make artifact downloading more robust for flaky networks" (#13995)"

This reverts commit 33d8a035.

* Use subdirectories to organize cached files

* Fix unauthorized import

* Revert "Change github readme links to be served by github (#13929)"

This reverts commit fd516d95.

* Revert " Rename SemanticsFlags to SemanticsFlag (#13994)"

This reverts commit 24e3f705.

* Revert "Make artifact downloading more robust for flaky networks (#13984)"

This reverts commit 4d37e03e.

* First version

* Add a clarifying comment

* Add verify to ZipDecoder.decodeBytes call

* Review comments

* Need OS Error for missing files

* FileSystemException, not OSError.

Add support for configuring the base storage URL for Flutter's
artifacts. If FLUTTER_STORAGE_BASE_URL is set, use it instead of
storage.googleapis.com.

The pub server can be overridden by setting PUB_HOSTED_URL.

Rather than depending on android.googlesource.com for hosting the gradle
wrapper, host our own copy on storage.googleapis.com (along with our
other artifacts).

Also added a script to repackage AOSP's version into ours, in case we
ever need to update it.

Addresses the gradle wrapper part of #11681.

* Add capability to print engine dart sdk to flutter doctor

* First round of review comments

* second round of comments

* Final review comments

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

* fix checkLockAcquired: support re-entrant locking

* add test; address comments

* add comment

* 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

* improve the logger ui a bit for elapsed times

* invert param

* update daemon.dart

The zip files downloaded from cloud storage are not clearly separated
into engine artifacts and tools anyways.

Also, because of this unclear separation, we were downloading
`linux-x64/artifacts.zip` twice before.

...now that we have generic methods, their types need to be specified too.

* Update to Dart SDK `1.22.0-dev.6.0`.

* Update SDK summary generation to create spec and strong summaries.

* Updated to dev.8.0.

* Fixed console out.

This ensures that accidental usages of dart:io's
file API don't creep in over time.

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

- [x] Remove old benchmark code from the non-hot runner.
- [x] Remove some dead code.
- [x] Fix a throw that should be a rethrow.

* make showElapsedTime default to true

* support nested progresses

* improve hot reload message

* rethrow