Fixes #3605

This patch addresses late-breaking comments on previous patches.

We were returning instead of breaking out of the loop when we hit the last
widget.

Fixes #3593

Cassowary doesn't have any additional dependencies and this simplifies things.

Fixes #2442

Some inherited widgets want to call this function direction, for example to
notify children when a mutable model object changes. Exposing this function to
subclasses of InheritedWidget is clear then forcing them to compute
updateShouldNotify for mutable model objects.

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

Rather that importing `package:newton/newton.dart` you can
`import package:flutter/physics.dart`.

Fixes #2441

These uses cases are now address by http.dart via http.readDataPipe.

* Fixed TimePicker crash and added tests

* Fixed TimePicker crash and added tests

* fixed a Lint-O

* move the material_gallery/demos.dart script

* rename all_demos.dart to all.dart

* add import guidance to library-level docs

* add import docs to other packages

* fix review comment

* clarify which libraries aren't meant to be directly imported

This patch adds an option to Scaffold to disable resizing the body to avoid the
window padding. This lets developers create layouts that are stable when the
keyboard overlays the app.

Fixes #3565

We had a math-o.

Fixes #3563

* add google analytics

* send in the run target type

* track device type targets

* use the real GA code

* review comments

* rev to usage 2.0

* rev to 2.2.0 of usage; add tests

* review comments

Wait until the end of the microtask to tell gesture recognizers that
they've won in the gesture arena. This lets recognizers dispose reject
themselves at arbitrary times without triggering gestures in awkward
call stacks.

Fixes #3183

* add a benchmarking mode to flutter analyze

* change arg names

Fixes #3186

Fixes #3289

Previously we were locking down the state even when calling layout in
LazyBlock. Now we lock only when building children. Making this work well
involved moving the catch out of lockState and into the few callers who
actually wanted it.

Fixes #3534

Handle the case of a null body in response.dart

We're supposed to return a null string when the HTTP response doesn't have a
body. Also handle the case of not having a headers map.

This is currently hidden for development use.  It will select artifacts from
the android_{Debug,Release}_Deploy output in a local engine build.

Improve docs for MediaQueryData.size

* improve text for devices

* nit

* tweak text

* exclude files from build/ when looking for asset variants

* exclude some dirs when creating flx files

This would have answered a question we go on flutter-dev.

Fixes #3517

Add Flow layout

A flow layout is optimized for reposition children with transformation
matrices. A flow layout can animate the position of its children very
efficiently.

This patch contains a few minor improvements to the framework that I ran
across while working on a new layout model.