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

Fixes #2442

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

* move the material_gallery/demos.dart script

* rename all_demos.dart to all.dart

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

The new names are consistent and clear.

* Adds NineSliceSprite and optimizes gallery front page

This reorders some classes so that this file makes more sense, and adds
a bunch of docs. It also makes the following changes:

* Move allElements from Instrumentation to TestWidgets. (Instrumentation
  is going away.)

* Remove findElements.

* Rename byElement to byElementPredicate

* Rename byPredicate to byWidgetPredicate

* Implement _WidgetPredicateFinder so that byWidgetPredicate has good
  messages

* Fix one use of byElementPredicate to use byWidgetPredicate.

Previously the widgets layer only provided a concrete binding, which
makes it awkward to extend it compared to other bindings. This moves
widgets to the same style as the other layers.

In a subsequent patch I'll use this to make the tests layer saner.

Makes the tests noisy.

Use the route table to generate the list of screens to test in the smoke test.

* Gallery Smoke Test

Previously this readme had some broken links.

Fixes #3428

* Make IconButtons as big as possible

The 'routes' table is a point of confusion with new developers. By
providing a 'home' argument that sets the '/' route, we can delay the
point at which we teach developers about 'routes' until the point where
they want to have a second route.

* Gallery code snippets now analyzed

* Remove Scaffold scrollable_key demo usage

* find all repo packages

* .dartignore

+ 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.

* Improvements to the Grid list demo

* More nits and fixes to grid demo in gallery

Current numbers from my Nexus 5:
  "average_frame_build_time_millis": 22.6854,
  "missed_frame_build_budget_count": 15,
  "frame_count": 15

Filed https://github.com/flutter/flutter/issues/3296

@yjbanov @hansmuller @vlidholt

...by adding tests to our examples that don't import flutter_test, which
pins the relevant dependencies.

Also, provide more information when complaining about leaked transient
callbacks in tests.

Also, make tests display full information when they have an exception,
by bypassing the throttling we have for Android logging in tests.

Also, make the word wrapping not wrap stack traces if they happen to
be included in exception output.

Also, fix a leaked transient callback in the checkbox code.

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

See https://github.com/flutter/flutter/issues/3233

Now past halfway though material.dart by files.

* ClampOverscrolls Inherited Widget

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.

* Added OverscrollIndicator, removed OverscrollIndicatorPainter