* round 3

* partially address comments

* merge

* review notes

* review

* review

* review

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

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

This reverts commit 8d07d3f6.

* fix some call sites of printError

* review comments

Bare bones widget inspector support.

Toggle the widget inspector from the flutter tool by pressing 'i'.
When the widget inspector is select mode:
Pointer down to to inspect a widget.
Pointer click to finalize selection of a widget. You can now interact
with the application as you normally would but with the inspected widget
highlighted.
Click the inspect icon in bottom left corner of screen to reactivate
select mode.

* remove the flutter view cache; fix the ability to quit the app after a full restart

* improve test

* update test

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

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

* review

* Remove '\n' from terminal input.

* Use trim instead of replaceAll

* Add unit test

* Cleanup the test

* Fixed lint

* Style adjustments

* Forgotten @override

* Revert "Forgotten @override"

Accidently added extra files.

This reverts commit 0aba24fc8ea321b3a4d0cd8aed7f589378393d96.

* Just @override change

* first pass

* improvements

* extract terminal.dart

* rebase

* add default terminal to context

* The analyzer wants the ../ imports in front of the ./ imports

* review notes

- [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

* Fix tests to use Ahem, and helpful changes around that

- Fix fonts that had metric-specific behaviours.

- LiveTestWidgetsFlutterBinding.allowAllFrames has been renamed
  to LiveTestWidgetsFlutterBinding.framePolicy.

- LiveTestWidgetsFlutterBinding now defaults to using a frame policy
  that pumps slightly more frames, to animate the pointer crosshairs.

- Added "flutter run --use-test-fonts" to enable Ahem on devices.

- Changed how idle() works to be more effective in live mode.

- Display the test name in live mode (unless ahem fonts are enabled).

- Added a toString to TextSelectionPoint.

- Style nit fixes.

* Roll engine to get Ahem changes.

* Update tests for dartdoc changes.

* Fix flutter_tools tests

Related #8876

Fixes #7307

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.

* Make ProcessSignals portable

This removes the need to wrap unsupported signals with in `if (!platform.isWindows) ..`.

It also allows us to implement a work around for breaking the Windows console when flutter is exited with Ctrl+C.

* review comments

* adding tests

* add license header

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.

* 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