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

Do not throw if a hot reload is attempted when all Flutter instances on the device have been stopped (#11504)

This can happen if an Android host app has been paused but its process is still running

* ignore postpauseevents when reloading

* Update run_hot.dart

comment changes to kick the appveyor bot

Fixes https://github.com/flutter/flutter/issues/11314

Changes introduced in
https://github.com/flutter/engine/commit/8ba522eeae35d8c70ada3c7b8e200ca2274f4f95
has removed from the `_flutter.listViews` the thread synchronization
side effect used during benchmarks.

The thread synchronization is restored via the new
`_flutter.flushUIThreadTasks` RPC.

Fixes https://github.com/flutter/flutter/issues/11241

Related https://github.com/flutter/engine/pull/3898

In
https://github.com/dart-lang/sdk/commit/df8bf384eb815cf38450cb50a0f4b62230fba217
a new functionality of the Dart VM Service Protocol has been introduced.

Clients connected to the Service Protocol are now able to expose
services that other clients (e.g. Observatory) can invoke through the
Service Protocol itself.

With these changes Flutter Tools register them self as a `reloadSources`
(a.k.a. HotReload) capable client.
Observatory is already listening for the clients which expose this
functionality and uses by default the service based version of
`reloadSources` when available, so requesting a HotReload from
Observatory will trigger the full Flutter HotReload.

Related https://github.com/dart-lang/sdk/issues/30023
Related https://github.com/flutter/flutter/pull/11229
Related https://github.com/flutter/flutter/pull/11256

This reverts commit 35c49897.

* Register tools as a reloadSources service

* Fix code style and comments

* Fix nits and Add documentation

* Format error message

Discovered dead code during review of #10791

* fix an issues with reloading when paused at a breakpoint

* update docs

`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

- [x] Switch the reassemble timeout to 5 seconds.
- [x] Print a status message if reassemble fails:

```
Performing hot reload...
Reassembling app.flx$main took too long. Hot reload may have failed.
Reloaded 0 of 418 libraries in 5,322ms.
```

Fixes #9316
Fixes #8861
Fixes #8857
Fixes #8856

- [x] Refresh FlutterViews before issuing a hot reload / restart

Fixes #7248

- [x] Resume the isolate before performing a hot restart.

Fixes #8923

- [x] Catch SocketErrors and handle them gracefully.
- [x] Print 'Lost connection to device' when the service protocol connection is severed unexpectedly.
- [x] Print 'Application finished' when the application exits otherwise.

After this PR:

```
Launching lib/main.dart on Nexus 7 in debug mode...
Running 'gradle assembleDebug'...                     1.2s
Built build/app/outputs/apk/app-debug.apk (21.7MB).
Syncing files to device...
Application finished.
DevFS sync failed. Lost connection to device: SocketException: OS Error: Connection refused, errno = 111, address = 127.0.0.1, port = 53062
Could not perform initial file synchronization.
```

Fixes #6705

The first hot reload does a bunch of work that we used to hide behind the
loader screen. This PR changes the messsage printed to the user on the first
reload from:

'Performing hot reload...'

to:

'Initializing hot reload...'

Subsequent reloads say 'Performing hot reload...'

Fixes https://github.com/flutter/flutter/issues/6849

* 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.

Function keys don't work great on any platform we support:
* Mac doesn't have first-class function keys.
* On Ubuntu: F1 opens the system help and F10 opens the file dialog.
* ... and Windows is a mess as well.

It was resulting in weird situations where the tool would dump an
error message and stack but not quit, or would fail hard but then just
hang.

Instead, specifically catch errors you expect. As an example of this,
there's one error we expect from the DartDependencySetBuilder, so we
catch that one, turn it into a dedicated exception class, then in the
caller catch that specific exception.

* Rename `Device.platform` to `Device.targetPlatform` to avoid
  collision with the exported variable from `platform.dart`

This implements the `DartDependencySetBuilder` completely in Dart instead of calling out to `sky_snapshot` (Linux/Mac) or `gen_snapshot` (Windows) and allows us to use the same code path on all supported host platforms.

It also slightly reduces hot reload times on Linux from ~750ms to ~690ms for the unchanged flutter_gallery app and significantly reduces hot reload times on Windows from almost 1.5s to just slightly slower than on Linux.

This change will also allow us to retire `sky_snapshot` completely in the future.

* [devFS] Use URI to represent paths on device

Previosuly, regular file paths in the format of the host platform were used to represent paths on device. That works when host and device share the same (POSIX) file path format. With a Windows host, this breaks. URIs are the solution as they are platform independent and the VM service on the device already interpreted the file paths as URIs anyways.

* review comments

* switch to file paths

* fix tests on Windows

* review comments

Instead, actually exit the tool.

Fixes #8363.

* Revert "Revert "Simplify path handling logic in dependency checker and devFS (#8414)" (#8467)"

This reverts commit 96ba7f76.

* Intentionally use a self-package URI in flutter_gallery

* tests to catch problems with self-package imports

This reverts commit e7bde11c.

Reason: broke hot reload when using "package:" style imports for sources
within the same project.

* Simplify path handling logic in dependency checker and devFS

Simplification will make it easier to port this to Windows.

* Roll Engine to 0a7b177c330367904597a6129b3eb653d29dfca0