* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

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.

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

Also:

 * Provide a better message when you lerp from infinity to finity
   constraints.

 * Make the striped marker support RTL.

 * By popular demand, dump a warning to the console the first time
   a particular Flex overflows. (Resets on hot reload.)

Fixes #11855

* Introduce a Directionality inherited widget which sets the ambient LTR vs RTL mode (defaulting to null, which means you cannot use directionality-influenced values).

* Make it possible to configure Padding (including Container.padding and Container.margin) using a directionality-agnostic EdgeInsets variant.

* Provide textDirection and verticalDirection controls on Row and Column to make them RTL-aware.

* Introduce a variant of FractionalOffset based on the EdgeInsets variant. Not yet actually used.

* Fix all the tests that depended on Row defaulting to LTR.

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments