* 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.

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.

* [driver] "waitFor" command in place of broken "exits"

* [driver] wait using frame callback

* rename the --develop option to --debug

* fail if both --debug and --deploy are specified

add explicit paths to the flutter recipies

Analyze CLI option to specify a custom Dart SDK.

Handy for testing against specific local SDK builds.

(Note that the option is hidden.)