* 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.

* improve progress display when running apps; speed up startup

* review comments

* comprehensive list of lints
* add comments to commented out lint
* fix unnecessary_this lints
* exclude prefer_final_fields

These tests have recently been turning the build red incorrectly. Until they are reliable, let's mark them flaky.

A change in overflow mode requires calculation of the text size and resetting
the overflow shader

Subsequent operations like debugAssertDoesMeetConstraints() may do layout
operations that alter _textPainter's state.

Fixes https://github.com/flutter/flutter/issues/9372

* Remove FadeTransition from _MountainViewFadeTransition.
* Update tests.