I can't figure out if this is genius or a giant hack.

This lets you use DefaultTextStyle.merge and IconTheme.merge without
specifying a BuildContext. It automatically merges in at the
appropriate place in the tree using a Builder widget.

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

These symbols were not intended to be public. Also, remove some bogus dartdocs
for PhysicalModeLayer.

* prefer const constructors

* fix double throw

Extend app bar left to edge, right to 4dp
Make leading button square and 56dp
Keep title at 72dp on Android according to Material
Renamed IconButton.size to .iconSize
IconButton minimum size expands to 48dp (#8264)
IconButton default constraints to 48. Can still stretch to infinity but can't be smaller than 48.
Ink splash for IconButton 40% bigger than the touch target to match Material
Tests

Answers http://stackoverflow.com/questions/41729278/why-is-this-icon-being-clipped

Previously, these classes where in material.dart, which meant they could
access private interfaces in that library. This patch moves them out
into their own files so that they need to play by the rules of the
public API.

Fixes #5969

Based on observations of usability study participant P1.

Fixes https://github.com/flutter/flutter/issues/5763

Using IconButton inside of a bigger Material before would result
in having a hit box as large as the whole Material. This commit
constrains the size of the hit box and splashes to the default
diameter of a splash.

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.

We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.

Fixes #287

Also:
* Make PaginatedDataTable able to scroll itself horizontally.
* Make drop down buttons support having an explicit text style and icon
  size given.
* Fix a bug with drop-down buttons asserting when opened partly
  off-screen.
* Make sure to pop the drop-down button's route if the drop-down button
  is discarded while the route is up.
* Remove extraneous padding on drop-down buttons. (Couldn't figure out
  why it was there, and it breaks alignment when a drop-down is mixed
  with other text.)
* Some docs improvements.
* Add Route.isActive
* Add a setState() method to ModalRoutes.

For consistency.

Fixes #4142

* Make IconButtons as big as possible

Now past halfway though material.dart by files.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Fixes https://github.com/flutter/flutter/issues/2369

This enum doesn't make sense anymore now that we can arbitrarily colorize
icons.  Instead, we just use a Color, which is both simpler and can be
interpolated during animations.

Fixes #1279

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

Fixes https://github.com/flutter/flutter/issues/1331

Hopefully this will be enough to start with. If this continues to be a
point of confusion we can see about adding more documentation or
changing the API somehow.

Now you can colorize an Icon simply by proving a Color for the Icon.

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

The right fix here is to use a radial reaction, but for now we can just make
the gesture detector including the padding acround the icon in its hit test
region.

Fixes #575

Now a RenderBox is considered hit if one of its children are hit or it itself
decides that it's hit. In particular, empty space inside a flex won't be hit
because none of the children are located there and a RenderFlex doesn't
consider itself hittable.

Fixes #53
Fixes #1221

The name of the parameter wasn't consistent with IconButton or DrawerItem, etc.

Fixes #1871

Material design icons are defined to work at specific sizes: 18, 24, 36, 48.
The current API doesn't reflect that and just takes a size int. If an invalid
size is chosen an error is printed to the console and no icon shows up.

Fixes #1816

Previous these callbacks were leaking the implementation detail that they were
triggered by taps. In a later patch, we're going to add a parameter to
GestureTapCallback that these callbacks won't have.

Related to #1807

Provide a way to schedule a callback for immediately after the current
frame has been sent to the GPU thread. This is useful for times where
you want to immediately schedule yourself for another build or layout
because, for instance, you just displayed frame zero of an animation and
you want to use the metrics from that layout to set up the actual
animation to begin immediately, such that the very next frame is frame 1.

Instead of requiring clients to use ui.ColorFilter, we show show the
ColorFilter class, similar to what we do for Color.