This patch migrates iOS device listing from using Xcode instruments to
using the libimobiledevice tools idevice_id and ideviceinfo.

ideviceinfo was previously incompatible with iOS 11 physical devices;
this has now been fixed.

In 37bb5f13 flutter_tools migrated from
libimobiledevice-based device listing on iOS to using Xcode instruments
to work around the lack of support for iOS 11. Using instruments entails
several downsides, including a significantly higher performance hit, and
leaking hung DTServiceHub processes in certain cases when a simulator is
running, necessitating workarounds in which we watched for, and cleaned
up leaked DTServiceHub processes. This patch returns reverts the move to
instruments now that it's no longer necessary.

* rename Element.visitChildrenForSemantics to debugVisitOnstageChildren

* address comments

* Improve the Flex overflow marker style

* Update basic.dart

* Update Hot Reload label text in template app instructions.

* label fix

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

* Revert "Revert "fire service protocol events for frames (#11565)" (#11727)"

This reverts commit f25e2f52.

* move the postEvent() call into a separate method

Fixes #11373

This patch includes:

 * SliverPadding
 * ScrollView
 * PageView

Fixes #11764
Fixes #11836

* fix typo in State.didUpdateWidget

* rephrase

* upgrade to linter-0.1.35

* re-enable lint prefer_const_constructors

* address review comments

Fixes #11374

When scrolling vertically, the scrollbar is on the left in RTL.

* ensureVisible works with viewports that have a center child

* review feedback

Fixes #11369

Fixes #11358

Fixes #11389

Fixes https://github.com/flutter/flutter/issues/11817

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

This patch adds Positioned.directional and PositionedDirectional. The
former is useful when you already know the TextDirection. The latter is
useful when you want to use the ambeient Directionality.

Fixes #11390

* Roll engine forward

* Update engine dep one commit further

* Update engine dep one commit further

Extract a Snapshotter class that can be shared between FLX snapshotting,
AOT snapshotting, and assembly AOT snapshotting. Allows for better
testability of snapshotting logic.

* Extracts script snapshotting used in FLX build.
* Adds tests for snapshot checksumming, build invalidation/skipping.

Remaining work: disentangle + extract AOT snapshotting and Assembly AOT
snapshotting logic from build_aot.dart.

Since the called function can't throw, there is no need for the "guarded".
Since the function returns something, running in guarded mode doesn't really work. The `uncaught` handler wouldn't know what to return (except for `null`).

* Notify the completer after the close animation completes on the snackbar

* Review comments

* Fix tests and analyzer warnings

* Fix analyzer warnings

In some cases, the element tree is not clean but we are required to dump the tree anyway.
To avoid crashing in those cases, we return an explicit null node.