* IconButton is a button, semantically

* fix datepicker test

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Most of the framework widgets should work in terms of EdgeInsetsGeometry
rather than EdgeInsets so that their clients can supply directional
insets.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

I added some tests for the bug that I fixed.
I added docs for IconButton and AppBar.
I added some new constructors for FractionalOffset.

Moved everything icons related except the material icon button and the materialicon font list to widget

Also fixes a small typo.

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