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

This reverts commit 8d07d3f6.

* Guess sign with the first certificate when multiple are available in machine mode

* review

`adb` can sometimes hang, which will in turn hang the Dart isolate if
we're using `Process.runSync()`. This changes many of the `Device` methods
to return `Future<T>` in order to allow them to use the async process
methods. A future change will add timeouts to the associated calls so
that we can properly alert the user to the hung `adb` process.

This is work towards #7102, #9567

* improve progress display when running apps; speed up startup

* review comments

Aggressively apply the const lint.

* 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.