Fixes https://github.com/flutter/flutter/issues/6690

Also add a version of the Gallery smoke test that enables semantics

This match switches the framework to use the semantics backend in `dart:ui`
rather than the Mojo backend.

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

This change might make our grammar less perfect, but it will increase our
sanity.

* Bump Dart SDK to `1.20.0-dev.10.0`.

`1.20.0-dev.10.0` corresponds to `1.20-RC0`.

* pubspec.lock cleanup

* check for pubspec.lock existence

* cleaned up conditional remove

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

This rewrites imports of various mojom.dart files from the Flutter
engine repo to instead import normal-looking dart files from the
(new) flutter_services package. This package handles exporting the
correct symbols from generated code wherever that may live.

Includes an engine roll to 3551e7a48e2e336777b15c7637af92fd7605b6c5
which contains the new flutter_services package.

The engine now reports coordinates in physical pixels.

I forgot that it was possible for the root view to get marked dirty
without getting a new widget. This fixes that case to work.

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

* General improvoments to the loader app:
   * Show a message after 8 seconds if no connection comes in.
   * Show a progress bar as files are being uploaded.
   * Hide the spinner just before launching the application.

* General improvements to the "flutter run" UI:
   * Add "?" key as a silent alias for "h".
   * Make the help text bold so it doesn't get mixed with the logs.
   * Make "R" do a cold restart when hot reload is enabled.

* Supporting features and bug fixes:
   * Add support for string service extensions.

* Other bug fixes:
   * Expose debugDumpRenderTree() outside debug mode.
   * Logger.supportsColor was missing a getter.
   * Mention in the usage docs that --hot requires --resident.
   * Trivial style fixes.

- show the next error fully after a hot reload
- hide _AssertionError in stacks
- immediately rebuild after a reassemble, so that hit tests work
- catch errors when notifying global key listeners

* send ext.flutter.debugDumpApp; ext.flutter.reassemble

* expose debugDumpRenderTree

We now stop updating semantics when there are no remaining clients.

Also, make SemanticsOwner into a real class and use it instead of a static in
several places.

There are still many more left before rendering.dart is complete.

For consistency.

Fixes #4142

This makes it possible to substitute 'flutter run' for 'flutter test'
and actually watch a test run on a device.

For any test that depends on flutter_test:

1. Remove any import of 'package:test/test.dart'.

2. Replace `testWidgets('...', (WidgetTester tester) {`
      with `testWidgets('...', (WidgetTester tester) async {`

3. Add an "await" in front of calls to any of the following:
    * tap()
    * tapAt()
    * fling()
    * flingFrom()
    * scroll()
    * scrollAt()
    * pump()
    * pumpWidget()

4. Replace any calls to `tester.flushMicrotasks()` with calls to
   `await tester.idle()`.

There's a guarding API that you can use, if you have particularly
complicated tests, to get better error messages. Search for
TestAsyncUtils.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

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

The new names are consistent and clear.

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.

* add a service extension for repaint rainbow

* review comments

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.

We're getting back to the point where we have a bunch of foundation APIs
and it's getting confusing having them mixed with services/.

Fixes #2723

Putting this code in a separate library let's services that depend only on the
shell be clearer about their dependencies.

This makes it match the material spec more.
https://www.google.com/design/spec/style/icons.html

Fixes https://github.com/flutter/flutter/issues/1357

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

Port the tests to use the new interface.

These were introduced by the recent engine roll.

Also, some trivial fixes for things that I found while playing with
the rendering library directly.

And rootConstraints to configuration.

This type and its variable have nothing to do with Constraints.

Fixes https://github.com/flutter/flutter/issues/829