--preview-dart-2 should turn on --strong, --reify_generic_functions, --limit_ints_to_64_bits so that we get true dart 2.0 semantics and performance

[a-siva]

The --preview-dart-2 option in 'flutter engine' and 'command line dart' should turn on
--strong
--reify_generic_functions
--limit_ints_to_64_bits
So that we get the true 2.0 semantics and performance.

[a-siva]

This also requires updating the version of Dart in flutter/flutter to a newer SDK (2.0 version).
Currently doing that produces a number of errors, there is a pull request out to fix some of these errors flutter/flutter#13223

[a-siva]

Two pull requests are currently out for review to address this -
flutter/engine#4504
flutter/flutter#13859

Landing these two will enable the ability to run flutter apps in strong mode.
There are still errors which need to be fixed in the flutter code to make it strong mode clean.

[a-siva]

The two PRs have landed and it is now possible to run flutter code in strong mode using flutter tools

flutter build flx --preview-dart-2 --strong
flutter run --preview-dart-2 --strong
flutter build aot --preview-dart-2 --strong

Flutter applications still fail when run in strong mode because of other issues, we now have a meta
issue at #31789 to track all other blocker issues that are stopping flutter from running in strong mode.