* Roll engine to d6d4eec24c38bb06a9b1aef4e70c321d447ba762

Contains the following engine commits:

flutter/engine@1bc0e1b On windows, refer to Dart snapshot directly executable.
flutter/engine@cf6ca32 Revert "Roll Dart to fe606f890b0a311da802c78b0af414a3c2087a79"
flutter/engine@4b45a53 Only make ERROR and FATAL log levels visible by default.
flutter/engine@98f6c2d Fix shell launcher test (dart2 compliant and stop polling)
flutter/engine@75851f0 Roll Dart to fe606f890b0a311da802c78b0af414a3c2087a79
flutter/engine@9495a52 On iOS, try to use ES3, then fall back to ES2.
flutter/engine@d6d4eec Fix broken Linux build
flutter/engine@d15dc76 Fix broken Chromebot build
flutter/engine@c17a629 Breadcrumbs to the wiki
flutter/engine@4eaf2c2 Return raw (unencoded) bytes in Image.toByteData()
flutter/engine@d812a61 Roll src/third_party/skia/ e669bdfad..276886160 (9 commits)
flutter/engine@62aeab7 Call SkSurface::flush instead of getting a backend handle in vulkan_swapchain.
flutter/engine@f4ebb18 Roll src/third_party/skia/ 3462eb034..e669bdfad (54 commits)
flutter/engine@0b7d6be Re-land "do not pause rendering when android view loses focus"
flutter/engine@91dee59 If the rasterizer has a valid surface context, use that to create a snapshotting render target.
flutter/engine@570231b Specify the packages file path when running engine dart tests.
flutter/engine@0b9cef5 Minor update to README
flutter/engine@c386418 Promote line height in text style created from paragraph style
flutter/engine@4e0fbb6 If the test specifies a .dill file, dont make the engine interpret is as source.
flutter/engine@58e84c8 Re-land "Support multiple shells in a single process.
flutter/engine@37e5df0 Remove only use of SkPaint::kGenA8FromLCD
flutter/engine@b7358b3 libtxt: cache font families that are remapped to the default font family
flutter/engine@a530035 libtxt: fix leaks in Skia object reference counting
flutter/engine@0c74fc9 [fuchsia] Fix use-after-free
flutter/engine@b6d2dde [fuchsia] Fix build
flutter/engine@1dd1264 [async] Adapt to libasync API changes.

* Run test platform finalizers in reverse order to ensure that shell is shutdown before trying to delete any temporary files or directories. This fixes "Access Denied" error on Windows (see #16657).

The engine expects it to always be there.

* add localizations for navigation menu, popup menu, dialog, and alert

Support exposing the InspectorService over the Flutterservice extension protocol as well as the observatory protocol. (#15876)

* Support exposing the InspectorService over the Flutter
service extension protocol as well as the observatory protocol.

We will probably remove most of the observatory protocol support once a
couple versions of the Flutter IntelliJ plugin have shipped that use the
Flutter service extension protocol. The only reason to continue supporting
the observatory protocol is it will allow using the inspector when paused
at a breakpoint.

This reverts commit b0b2c007.

* Turn support for sync-async on in Flutter. With this feature
execution of async functions starts immediately instead of it
being delayed by one microtask tick.

* Address review comments.

* Adjust some stack trace expectations which are slightly different with
--sync-async turned on.

We don't really like the name AnimatedChildSwitcher, and we think that AnimatedSwitcher might be better (since the Child part is redundant). We also considered AnimatedChild, AnimatedTransition and AnimatedReplacement (among others).

Nothing in here besides a rename.

* Don't return null, just rethrow RpcException error when vmservice responds with error.

Revert some uses of Number.tryParse(), ignore deprecation warnings for Number.parse()

* Handle error count reported by frontend.

Extend compilation result from single string to a structure(string filename and integer error count).

* Use ?.

* Include engine roll with dart sdk roll.

* parse(onError) -> tryParse

* Make '?? throw' more readable and avoid issue with analyzer

* Fix test so it mocks compiler output including errors count

This fixes a rendering problem in the AnimatedChildSwitcher where it would add a new "previous" child each time it rebuilt, and if you did it fast enough, all of them would disappear from the page.

It also expands the API for AnimatedChildSwitcher to allow you to specify your own transition and/or layout builder for the transition.

Fixes #16226

This reverts commit 6494ab23f6a063c689441f55c3aba1b3c8e26b5b.

* fix a test flake on appveyor

* only catch FileSystemExceptions

* A new UI is being designed to make the unused reload messaging clearer,
the UI will use the coverage data to highlight lines in the UI that were not executed during a reload.

In lieu of that we are turning off the unused reload messages experiment.

There were some issues in Dart2 also with this functionality
- source fingerprinting is not implemented in Dart2
- some additional synthetic functions are generated for invocation argument checking that are not appropriately filtered out.

This converts the chips to use a ChipThemeData to get most of their customization values from (if not overridden by specific arguments to the chip constructors), and to have the base ThemeData contain one of these. It also adds the ChipTheme widget that will allow overriding the theme for a particular subtree in the widget hierarchy.

Added tests for both, and just more tests in general for the Chips.

The spec changed the size of the slider thumb, so this just effects that change.

When flutter builds are run with --verbose, pass the flag through to
flutter sub-commands run via xcode_backend.sh.

* Introduce fingerprint check for preview-dart-2 build flx.

* Don't count outputs into fingerpint. Refactor compilation code.

Inlines the very small amount of work being done in _build() into
buildScriptSnapshot(). Eliminates a duplicate (and un-awaited) call to
_writeFingerprint.

Revert "re-write flutter analyze (the single-shot and --flutter-repo) to use the analysis server (#16281)" (#16482)

This reverts commit 2f41ea54.

re-write flutter analyze (the single-shot and --flutter-repo) to use the analysis server

* Sync .packages file as regular file.

Currently .packages file is treated specially as DevFSStringContent to accommodate package-file rewrite when it is sent to the device for dart1 compilation. In dart2 we need to treat .packages as regular file because from frontend perspective it's just a normal input file.

Revert 9749bea7. Accidentally pushed to flutter instead of fork repo for PR.

This reverts commit 9749bea7.

Fixes https://github.com/flutter/flutter/issues/16346.

See https://github.com/flutter/flutter/issues/16346#issuecomment-380255095 for detailed explanation of what was going wrong before this fix.