This patch uses sliver-based two more gallery demos, the stocks example,
in the date picker, and in markdown.

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.

See https://github.com/dart-lang/sdk/issues/28463, which I think is a
regression.

This also fixes the regression introduced by
https://codereview.chromium.org/2559773002 whereby we were no longer
checking any of the lints.

We expect TextField to be used much more often than Input. This patch updates
our old example code to use TextField instead.

See #7031

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.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

Removed old Stack layout and added a simple-to-extend interface for the
new drawer header. Also added a specialized UserAccountsDrawerHeader
consistent with Material Design guidelines.

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.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

Fixes 3544

TextAlign applies to a whole paragraph instead of applying to an individual
text span. This patch moves the property out of TextStyle and into a separate
property on Text and RichText.

Previously, the client of PopupMenuButton needed to build all the menu times

when building the PopupMenuButton. This can get expensive if, for example, each
item in a scrollable list has a popup menu associated with it.

Now the client passes a builder function to the PopupMenuButton that gets
invoked only when its time to show the menu items.

This matches the pattern from Rect. I've left EdgeInsets.TRBL marked as
deprecated to give clients a chance to update.

Fixes #2860

Previously, border with '0' was ambiguous. Sometimes we treated it as
hairline borders, sometimes as "don't show the border", though even in
the latter case we did some graphics work sometimes. Now we have an
explicit BorderStyle.none flag to not draw the border efficiently.

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

Fixes #231

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

Fixes #2348

Fixes #2353

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Fixes #1278

And fix the zillion issues that uncovered.