This prepares us for a CustomScrollView that takes slivers.

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.

This patch deploys ScrollView in a number of demos and manual tests.

...now that we have generic methods, their types need to be specified too.

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

We were trying to cram too much functionality in to the Dialog widget. Now we
have AlertDialog and SimpleDialog to cover to two different kinds of dialogs in
the spec.

Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.

Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.

Icon itself now takes the icon as a positional argument, for brevity.

ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.

IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.

IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.

IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.

Most places that introduced an IconTheme widget now use IconTheme.merge.

IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.

ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.

Also: Fix the analyzer to actually check the stocks app.

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.

This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.

Fixes #2313
Fixes #2218
Fixes #2009
Fixes #994

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

- Add an option to enable debugPaintSizeEnabled.
- Add an option to enable the StatisticsOverlay.
- Add support for the StatisticsOverlay in MaterialApp.
- Change the layout behaviour of RenderStatisticsBox:
   - Give it a zero intrinsic width.
   - Give it an accurate intrinsic height that depends on the flags
     set. (Also, move the enum to the rendering/ layer from the
     widgets/ layer to enable this.)
   - Make the box automatically size itself full-width and the correct
     height, so that you can actually embed it (though most of the
     time you'd just put it in a Stack so this doesn't matter as
     much, really).
- Some style nit fixes in statistics_box.dart.

This will make it much easier to add more settings.

See https://www.google.com/design/spec/layout/metrics-keylines.html#

Also, fill in a back arrow when we can go back.

Fixes #699

To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

It's common to want a scrolling viewport but with padding around the
contents. Teaching Block about this makes the places that do this
substantially simpler and further buries ScrollableViewport and
BlockBody (they're now only used in scrollable.dart).

This patch introduces the notion of a ModalRoute that puts up a modal barrier
and makes the route invisible to hit testing when its animating out. This patch
also uses this mechanism in a number of places (including PageRoute). There are
still a few more cases to convert, but that's work for a future patch.

Fixes #1684

- Change RouteArguments to pass the route's BuildContext rather than
  the Navigator. This caused the bulk of the examples/ and .../test/
  changes (those are mostly mechanical changes). It also meant I could
  simplify Navigator.of().

- Make initState() actually get called when the State's Element is in
  the tree, so you can use Foo.of() functions there. Added a test for
  this also.

- Provide a RouteWidget so that routes have a position in the Widget
  tree. The bulk of the route logic is still in a longer-lived Route
  object for now.

- Make Route.setState() only rebuild the actual route, not the whole
  navigator.

- Provided a Route.of().

- Provided a Route.writeState / Route.readState API that tries to
  identify the clients by their runtimeType, their key, and their
  ancestors keys, up to the nearest ancestor with a GlobalKey.

- Made scrollables hook into this API to track state. Added a test to
  make sure this works.

- Fix the debug output of GestureDetector and the hashCode of
  MixedViewport.

- Fixed ScrollableWidgetListState<T> to handle infinite lists.

Now you don't need to pass the navigator around everywhere.

Fixes #1669

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

Canvas is by far the most popular type of material by call site (and makes for
a faster default because it doesn't have a RRect clip).

Rather than using a custom render object, we can just use a Stack.

All the use cases for EventDisposition are now addressed by the gesture
detection system.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.

Mostly formating and removing arguments with default values.

These widgets were designed in CSS where you need to specify a layout model for
your children. This patch updates them to the modern style of just taking a
unique child.

Fixes #755

Block -> BlockBody
ScrollableBlock -> Block
FixedHeightScrollable -> ScrollableWidgetList
VariableHeightScrollable -> ScrollableMixedWidgetList
BlockViewport -> MixedViewport

This refactors Checkbox to own a RenderObject similar to how Switch was
refactored in https://github.com/domokit/sky_engine/pull/376 and
extracts common functionality for toggleable renderers into a base
class.  Switch and Checkbox's render objects derive from this base
class to add their own custom painting and theming logic.