A flow layout is optimized for reposition children with transformation
matrices. A flow layout can animate the position of its children very
efficiently.

This patch contains a few minor improvements to the framework that I ran
across while working on a new layout model.

Fixes #3510

Somehow I missed these in my previous patch.

Fixes #3503

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.

(Reland)

This adds in particular the ability to track the time at which the
framework boots up, and the time at which we are confident we have
completed the first useful frame.

Fixes #3501

We don't need this assert anyway.

This reverts commit 161f945e.

This patch caused a number of regressions.

Fixes #3497

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

* Refactored MaterialApp

* Remove IconThemeData.clampedOpacity

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.

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

Also fix selection changes on iOS.

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

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.

* 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.

* 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.

* 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.

Bindings now have a debugRegisterServiceExtensions() method that is
invoked in debug mode (only). (Once we have a profile mode, there'll be
a registerProfileServiceExtensions() method that gets called in that
mode only to register extensions that apply then.)

The BindingBase class provides convenience methods for registering
service extensions that do the equivalent of:

```dart
void extension() { ... }
bool extension([bool enabled]) { ... }
double extension([double extension])  { ... }
Map<String, String> extension([Map<String, String> parameters]) { ... }
```

The BindingBase class also itself registers ext.flutter.reassemble,
which it has call a function on the binding called
reassembleApplication().

The Scheduler binding now exposes the preexisting
ext.flutter.timeDilation.

The Renderer binding now exposes the preexisting ext.flutter.debugPaint.

The Renderer binding hooks reassembleApplication to trigger the
rendering tree to be reprocessed (in particular, to fix up the
optimisation closures).

All the logic from rendering/debug.dart about service extensions is
replaced by the above.

I moved basic_types to foundation.

The FlutterWidgets binding hooks reassembleApplication to trigger the
widget tree to be entirely rebuilt.

Flutter Driver now uses ext.flutter.driver instead of
ext.flutter_driver, and is hooked using the same binding mechanism.
Eventually we'll probably move the logic into the Flutter library so
that you just get it without having to invoke a special method first.

To be more consistent with other parts of the platform:

* put the binding in a binding.dart file.

* rearrange some members of the Scheduler class to be more close to
  execution order.

* factor out Priority class into its own file.

* add more dart docs.

+ Add new demo to gallery to show data tables. (This currently doesn't
use a Card; I'll create a Card version in a subsequent patch.)

+ Fix checkbox alignment. It now centers in its box regardless.

+ Add Colors.black54.

+ Some minor fixes to dartdocs.

+ DataTable, DataColumn, DataRow, DataCell

+ RowInkWell

+ Augment dartdocs of materia/debug.dart.

+ DropDownButtonHideUnderline to hide the underline in a drop-down when
  used in a DataTable.

+ Add new capabilities to InkResponse to support RowInkWell.

+ Augment dartdocs of materia/material.dart.

+ Add an assert to catch nested Blocks.

+ Fix a crash in RenderBox when you remove an object and an ancestor
  used its baseline. (https://github.com/flutter/flutter/issues/2874)

+ Fix (and redocument) RenderBaseline/Baseline.

+ Add flex support to IntrinsicColumnWidth.

+ Document more stuff on the RenderTable side.

+ Fix a bug with parentData handling on RenderTable children.

+ Completely rewrite the column width computations. The old logic made
  no sense at all.

+ Add dartdocs to widgets/debug.dart.

+ Add a toString for TableRow.

Fixes https://github.com/flutter/flutter/issues/3363

Fixes https://github.com/flutter/flutter/issues/3352

If two repaint boundaries mark themselves dirty, but the second one is a
child of the first, then the second one will get repainted by the first
and then when we come to paint it directly, we get confused because it
isn't dirty any more.

We ran into this in layout before. Apply the same fix. Also, apply the
same fix to composition while we're at it.

Exception stack variable was shadowing the function argument stack.