The old code had gotten crufty with all the refactors. This simplifies
it down to what it really means.

Also, add a bunch of tests.

We used to rely on BoxConstraints.deflate's clamping, but now we have to
roll our own.

We used to use this mechanism to paint scrollbars, but it's awkward. The
new approach to scrollbars is much cleaner. This mechanism has no other
clients, so we should remove it.

This makes it possible to substitute 'flutter run' for 'flutter test'
and actually watch a test run on a device.

For any test that depends on flutter_test:

1. Remove any import of 'package:test/test.dart'.

2. Replace `testWidgets('...', (WidgetTester tester) {`
      with `testWidgets('...', (WidgetTester tester) async {`

3. Add an "await" in front of calls to any of the following:
    * tap()
    * tapAt()
    * fling()
    * flingFrom()
    * scroll()
    * scrollAt()
    * pump()
    * pumpWidget()

4. Replace any calls to `tester.flushMicrotasks()` with calls to
   `await tester.idle()`.

There's a guarding API that you can use, if you have particularly
complicated tests, to get better error messages. Search for
TestAsyncUtils.

If there's a max height or width, we should factor that into the intrinsic
sizing for the other dimension.

Turns out a common use for Baseline is making sure that the child's
baseline is higher than it otherwise would be, e.g. with the Material
font or other symbols.

Fixes #3289

Fixes #3501

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

* Make IconButtons as big as possible

+ Add new demo to gallery to show data tables. (This currently doesn't
use a Card; I'll create a Card version in a subsequent patch.)

+ Fix checkbox alignment. It now centers in its box regardless.

+ Add Colors.black54.

+ Some minor fixes to dartdocs.

+ DataTable, DataColumn, DataRow, DataCell

+ RowInkWell

+ Augment dartdocs of materia/debug.dart.

+ DropDownButtonHideUnderline to hide the underline in a drop-down when
  used in a DataTable.

+ Add new capabilities to InkResponse to support RowInkWell.

+ Augment dartdocs of materia/material.dart.

+ Add an assert to catch nested Blocks.

+ Fix a crash in RenderBox when you remove an object and an ancestor
  used its baseline. (https://github.com/flutter/flutter/issues/2874)

+ Fix (and redocument) RenderBaseline/Baseline.

+ Add flex support to IntrinsicColumnWidth.

+ Document more stuff on the RenderTable side.

+ Fix a bug with parentData handling on RenderTable children.

+ Completely rewrite the column width computations. The old logic made
  no sense at all.

+ Add dartdocs to widgets/debug.dart.

+ Add a toString for TableRow.

If two repaint boundaries mark themselves dirty, but the second one is a
child of the first, then the second one will get repainted by the first
and then when we come to paint it directly, we get confused because it
isn't dirty any more.

We ran into this in layout before. Apply the same fix. Also, apply the
same fix to composition while we're at it.

* Fix Block intrinsic functions

* Update block_test.dart

Fix the test per travis

This also shrinks the width of the error messages a bit because now that
we use 'package:' URLs the stacks are a bit narrower.

I ran into a case where I was setting minHeight=∞ and then calling
layout() with that constraint, which is all kinds of bad. To try to
catch this earlier, this patch now provides a way to catch constraints
that are requiring infinite values.

We don't _always_ check this because there are valid uses for
BoxConstraints.biggest, e.g. as an additionalConstraint.

Previously this was used by MixedViewport, but now we don't need it because

LazyBlockViewport has replaced MixedViewport.



I've also taken this opportunity to modernize RenderBlock.

Fixes #2855

- Add x and y to the parent data, in case it's useful. In point of fact,
  I ended up not needing it.

- Make columnWidths settable rather than having a setter so that the
  code in the widgets layer is more idiomatic.

- Teach setFlatChildren about avoiding unnecessary drop/adopt loops.

- Assert that the child list length is correct more aggressively so that
  we catch the dumb bug I had more quickly next time.

- Improve the toString() to handle empty tables better.

- Resort the imports in framework.dart.

- Introduce a LocalKey for cases where you don't want to allow use of
  GlobalKey.

- Make inflateWidget and deactivateChild public for subclass use.

- Table widget.

- Cell widget.

- TableRow fake widget.

Adds some names for common FractionalOffset values.

Fixes #2723

New RenderAligningShiftedBox abstract class for render boxes with
"alignment", which as of now is:

 - RenderPositionedBox
 - RenderConstrainedOverflowBox (ex RenderOverflowBox)
 - RenderSizedOverflowBox
 - RenderFractionallySizedOverflowBox (ex RenderFractionallySizedBox)

(The respective widgets are Align, OverflowBox, SizedOverflowBox, and
FractionallySizedBox. They haven't been renamed.)

Also I added some toString details.

Fixes https://github.com/flutter/flutter/issues/2735

These were using Offsets, but they're really FractionalOffsets.

Fixes #2318

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

Fixes #1382

Fixes #774

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

Putting this code in a separate library let's services that depend only on the
shell be clearer about their dependencies.

Makes it easier to use without a SizeObserver

Otherwise, changing the order of your children (e.g. shuffling a list of
children) will have no visible effect.

One bug was masking the other, hence they both needed to be fixed
and tested separately.

@Hixie