* Adds NineSliceSprite and optimizes gallery front page

* Refactored MaterialApp

* Remove IconThemeData.clampedOpacity

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.

Roll engine to 4062813e7060666f26b89e77d3f58035523bfd6b

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.

Also simplify handle drawing. All 3 cases are the same, just rotated.

Also fix selection changes on iOS.

Add a service extension for toggling the PerformanceOverlay

Moves TestGesture into test_pointer.dart and makes it more
self-contained.

This is part of a general refactoring of flutter_test.

Depends on https://github.com/flutter/flutter/pull/3459

This only works for apps which use WidgetsApp.  Apps which don't
(like the game) could presumably read the static themselves
off of WidgetsApp.

@devoncarew @hixie

* Refactor the Gesturer's interfaces

This makes them more coherent.

It also makes it easier for the tests to override each specific part of
hit-testing, should that be necessary.

* Update binding.dart

* Enhance Finder toString()

Only partially works. Editing the selected text doesn't work very well,
which probably will require engine changes. Currently only draws the
selected text and allows you to manipulate the selection with draggable
selection handles.

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

* Gallery Smoke Test

Prior to this patch, we were pushing two frames each time the cursor blinked.
In turning the cursor on or off, the markNeedsPaint call was triggering another
frame to be scheduled because we cleared a bit in the scheduler at the
beginning of the frame instead of at the end of the frame.

To implement scheduling correctly, we actually need two bits: one for
ensureVisualUpdate, which just promises to get to the end of the pipeline soon,
and scheduleFrame, which promises to get to the beginning of the pipeline soon.

Also a bit of code cleanup.

The key part of this patch is the addition in `_endScroll` to reset
`_simulation`. It seems like this was the one place where it's possible
for us to end the animation but not reset our state. Since we assert
that are state is coherent, we were hitting asserts when a fling
finished and then you interacted with the widget again.

Previously this readme had some broken links.

Fixes #3428

* move driver create test to the flutter_tools package

* review comments

* 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

...so that people don't think it's for Row.

* allow flutter create to re-gen over an existing project

* add a regression test

Fixes https://github.com/flutter/flutter/issues/3149

* Remove Scaffold scrollable_key demo usage

This makes it easier to use e.g. AssetImage in simple test applications.

Also, dartdoc improvements.

Also, use @required in one place, to see if it causes any trouble. If it
doesn't, I'll start using it in more places.

* add a service extension for repaint rainbow

* review comments

The defaulting logic for bundles really belongs in the services layer,
not the widgets layer. This way we can tell non-widget code just to use
rootBundle if it just wants to read a JSON file or some such.