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

This reverts commit 8d07d3f6.

Gradle APK builds include engine binaries for all applicable architectures

* add @required when there's an assert not null

* address review comments

* Remove unnecessary Android SDK checks.

* Remove unused accessors, skip replay test.

* 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.

If a target file is specified on the flutter tools command line, pass it
through to Gradle.

It is still possible to statically specify a target file in the flutter
section of build.gradle, but it is now possible to specify it on the
command line as well. The command line option takes precedence.

Fixes #8175.

* Fix bug parsing Gradle version.

Version from pub_semver requires versions of the format X.Y.Z. Gradle
doesn't follow semantic versioning, though, so version parsing would
fail on versions like '3.2'. Fixed by writing a custom Version class.

Also removed a check for apksigner when building Gradle-based projects.

Fixes #8298

This reverts commit 70d7fe3a.

The Java version missmatch on the bots has been resolved.

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.

* improve the logger ui a bit for elapsed times

* invert param

* update daemon.dart

* Update gradle example to support x86 in debug mode.

Changed the Flutter Gradle plugin a bit to better fit in with the
Android build.

Fixes #6136
Fixes #6864
Fixes #7539

- [x] Wire up dependency checker and plumb flags down to the right place

Fixes #7014

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.

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions

* make showElapsedTime default to true

* support nested progresses

* improve hot reload message

* rethrow

Fixes #4166.
Fixes #5120.
Fixes #5399.
Fixes #6507.

Required for compatibility with older versions of Android

* refactor _run to runCmd

* replace requiresProjectRoot getter with call to commandValidator

* replace requiresDevice getter with call to findTargetDevice

* trace command requires a debug connection, not a device

* inline androidOnly getter

* rename command methods to verifyTheRunCmd and runCmd

* move common verification into BuildSubCommand

* rename deviceForCommand to device

* rename methods to verifyThenRunCommand and runCommand

Fixes https://github.com/flutter/flutter/issues/5592

This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.

Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.

This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.

* This reverts commit 5e7e7b6e.

* default to lib/main.dart

This reverts commit 32ad81c8.