This cleans up lerp, lerpFrom, lerpTo, and scale methods, and their
documentation.

Fixes https://github.com/flutter/flutter/issues/12377.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

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.)

Diagnostics object is.

* 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.

These properties had a high cost in the documentation because they're
all top-level properties, and there's really very little reason for
these values to be configurable in the first place.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* Optimise AnimatedSize for the tight case.
* Remove `default` from a switch statement over enum (so that analyzer will complain if we add enum values).
* Adopt the Size since we use it after the child may have changed (which would throw normally).
* AnimatedCrossFade.layoutBuilder

* Revert "Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)"

This reverts commit ceb814aa.

Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)

This reverts commit d46e208b.

* Change all instances of '$runtimeType#$hashCode' to use the describeIdentity method.

The describeIdentity method generates a shorter description with a consistent length
consisting of the runtime type and the a 5 hex character long truncated
version of the hash code.

Introduce CompositedTransformTarget and CompositedTransformFollower
widgets, corresponding render objects, and corresponding layers.

Adjust the way text fields work to use this.

Various changes I needed to debug the issues that came up.

* move getTransformTo method from RenderBox to RenderObject.

* Cleanup comment. Add slivers test.

This should improve debug-time performance.

This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

Previously we used logical pixels. This made the accessibility metrics
tiny on modern devices, since the OS works in physical units.

Also add a bit more debugging info and some docs.

Also, fix some logic in SliverPadding.

Also includes minor doc fixes.

* make @immutable const

* fix build

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

Turns out we have many immutable classes.

Fixes #6892

* Manually fix every use of Point.x and Point.y

Some of these were moved to dx/dy, but not all.

* Manually convert uses of the old gradient API

* Remove old reference to Point.

* Mechanical changes

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\bPoint[.]origin\b/Offset.zero/g'
git ls-files -z | xargs -0 sed -i 's/\bPoint[.]lerp\b/Offset.lerp/g'
git ls-files -z | xargs -0 sed -i 's/\bnew Point\b/new Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bconst Point\b/const Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bstatic Point /static Offset /g'
git ls-files -z | xargs -0 sed -i 's/\bfinal Point /final Offset /g'
git ls-files -z | xargs -0 sed -i 's/^\( *\)Point /\1Offset /g'
git ls-files -z | xargs -0 sed -i 's/ui[.]Point\b/ui.Offset/g'
git ls-files -z | xargs -0 sed -i 's/(Point\b/(Offset/g'
git ls-files -z | xargs -0 sed -i 's/\([[{,]\) Point\b/\1 Offset/g'
git ls-files -z | xargs -0 sed -i 's/@required Point\b/@required Offset/g'
git ls-files -z | xargs -0 sed -i 's/<Point>/<Offset>/g'
git ls-files -z | xargs -0 sed -i 's/[.]toOffset()//g'
git ls-files -z | xargs -0 sed -i 's/[.]toPoint()//g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point, /show /g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point;/show Offset;/g'

* Mechanical changes - dartdocs

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\ba \[Point\]/an [Offset]/g'
git ls-files -z | xargs -0 sed -i 's/\[Point\]/[Offset]/g'

* Further improvements and a test

* Fix minor errors from rebasing...

* Roll engine

Also, add a specific message for BuildContext.size for RenderSlivers.

The new scrolling world marks nodes dirty between frames instead of
waiting for the frame, and we've decided that's actually ok.

There's no test here because the test harness has a bug that hides
this exception. I will submit a separate PR to fix the harness, which
will fix a test that, without _this_ patch, fails. All of which is to
say, this is actually already tested.

These parameters are the width, not the height.

Fixes #4301

Covers lib/ in package:flutter.

Fixes #7734

These functions were previously hidden inside other functions but they
are useful enough to expose directly. Specifically, I plan to use these
when implementing Scrollable2.ensureVisible.