`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

Also:

 * Add three explicit sizing modes to Stack for non-positioned
   children: loose, expand, and passthrough. (All three are used.)

 * Fix a bug whereby layers would try to paint in the same frame as
   they were removed from layout (but not detached).

 * Fix a bug whereby Offstage wasn't properly marking the parent dirty
   when changing its sizedByParent flag.

 * Explicitly make Overlay expand non-positioned children.

 * Explicitly have InputDecoration pass through the constraints from
   its Row to its Stack children.

Fixes https://github.com/flutter/flutter/issues/8425

* Do not lower-case paths during canonicalization.

This breaks hot reload on some platfroms with case insensitive file systems.

* Add unit tests

This patch reworks some of the guts of scrolling to make it easier to
implement nested scrolling effects. The actually nested scrolling effect
will be included in a later patch.

This patch introduces the notion of a keyboard token, which generalizes the
logic in EditableText for distinguishing between gaining focus by default and
gaining focus because of an explicit use action.

Fixes #7985

* Remove Pods dir from layers example

* Remove Pods dir from flutter_view example

This will help us know for sure exactly which sub-script erred.

It's going to be replaced by #8963

Fixes #8947

* Upgrade node version

* Upgrade node version, but no further than 6 which is in LTS

* more widget const constructors

* prefer const constructors

* address review comments

Without this flag, it shows progress via increasing numbers instead of a progress bar. For some reason, the IDEA console seems to be okay with that.

* enable lint overridden_fields

* address review comment

This constructor makes it easier to use a GridView with a builder
callback.

Fixes #8952

* make @immutable const

* fix build

Fixes #5166

We now attempt to explain what went wrong.

Fixes #9509

Move the Drag*Details classes into drag_details.dart.
Move the one-gesture drag recognizers into monodrag.dart.
Move Drag into drag.dart.

Adjust the comments to claim that Drag is used by other things than
MultiDragGestureRecognizer. (Right now this is a lie but it will
hopefully be true soon.)

This is such a fundamental property of the system that we forgot to
document it.

Fixes #6818

* Have sanity check pass on old and new versions of flutter

* Add Pods cache dir to ios/.gitngnore

* Add missing ending newlines

* Make seperation between app and plugin main.dart more manageable.

* Jakob review feedback

* Remove empty line

* Add missing newline

There is a race where we could complete a future based on a stream event
and then before we cancel the stream subscription, we get another event
and try and complete the future again.