Also add dartdocs.

* Initial set of tests for flutter sprites

This currently requires a local build of the engine.

Run "flutter build aot" to build an AOT snapshot.
Run "flutter build apk --profile" to build a "profile mode" APK that uses AOT.

* Refactor widget test framework

Instead of:

```dart
  test("Card Collection smoke test", () {
    testWidgets((WidgetTester tester) {
```

...you now say:

```dart
  testWidgets("Card Collection smoke test", (WidgetTester tester) {
```

Instead of:

```dart
  expect(tester, hasWidget(find.text('hello')));
```

...you now say:

```dart
  expect(find.text('hello'), findsOneWidget);
```

Instead of the previous API (exists, widgets, widget, stateOf,
elementOf, etc), you now have the following comprehensive API. All these
are functions that take a Finder, except the all* properties.

* `any()` - true if anything matches, c.f. `Iterable.any`
* `allWidgets` - all the widgets in the tree
* `widget()` - the one and only widget that matches the finder
* `firstWidget()` - the first widget that matches the finder
* `allElements` - all the elements in the tree
* `element()` - the one and only element that matches the finder
* `firstElement()` - the first element that matches the finder
* `allStates` - all the `State`s in the tree
* `state()` - the one and only state that matches the finder
* `firstState()` - the first state that matches the finder
* `allRenderObjects` - all the render objects in the tree
* `renderObject()` - the one and only render object that matches the finder
* `firstRenderObject()` - the first render object that matches the finder

There's also `layers' which returns the list of current layers.

`tap`, `fling`, getCenter, getSize, etc, take Finders, like the APIs
above, and expect there to only be one matching widget.

The finders are:

 * `find.text(String text)`
 * `find.widgetWithText(Type widgetType, String text)`
 * `find.byKey(Key key)`
 * `find.byType(Type type)`
 * `find.byElementType(Type type)`
 * `find.byConfig(Widget config)`
 * `find.byWidgetPredicate(WidgetPredicate predicate)`
 * `find.byElementPredicate(ElementPredicate predicate)`

The matchers (for `expect`) are:

 * `findsNothing`
 * `findsWidgets`
 * `findsOneWidget`
 * `findsNWidgets(n)`
 * `isOnStage`
 * `isOffStage`
 * `isInCard`
 * `isNotInCard`

Benchmarks now use benchmarkWidgets instead of testWidgets.

Also, for those of you using mockers, `serviceMocker` now automatically
handles the binding initialization.

This patch also:

* changes how tests are run so that we can more easily swap the logic
  out for a "real" mode instead of FakeAsync.

* introduces CachingIterable.

* changes how flutter_driver interacts with the widget tree to use the
  aforementioned new API rather than ElementTreeTester, which is gone.

* removes ElementTreeTester.

* changes the semantics of a test for scrollables because we couldn't
  convince ourselves that the old semantics made sense; it only worked
  before because flushing the microtasks after every event was broken.

* fixes the flushing of microtasks after every event.

* Reindent the tests

* Fix review comments

These wrappers also give us a chance to document what these services do.

Fixes #3164

Turns out there were more clients of the old paragraph API than I expected.
This patch migrates them to the new API.

This fixes a bug where editing the selected text on Android would delete
the wrong block of text.

BUG=https://github.com/flutter/flutter/issues/3567

Turns out a common use for Baseline is making sure that the child's
baseline is higher than it otherwise would be, e.g. with the Material
font or other symbols.

Note that the /// part of the doc was published, only the // line was
omitted. This means that we can safely put // comments between /// docs
and the member they are documenting.

Instead of using properties, TextPainter now receives min and max width as
parameters to layout. Also, this patch integrates the intrinsic sizing logic
into the main layout function, which satisfies all the existing uses cases.

Apps that didn't use [WidgetsApp] were not sending the `'Widgets
completed first useful frame'` notification. This fixes that by making
the code cleaner.

Related to #3608

* simplify the analysis benchmarking code

* review comments

Fixes #3605

This patch addresses late-breaking comments on previous patches.

We were returning instead of breaking out of the loop when we hit the last
widget.

Fixes #3593

Cassowary doesn't have any additional dependencies and this simplifies things.

Fixes #2442

Some inherited widgets want to call this function direction, for example to
notify children when a mutable model object changes. Exposing this function to
subclasses of InheritedWidget is clear then forcing them to compute
updateShouldNotify for mutable model objects.

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

Rather that importing `package:newton/newton.dart` you can
`import package:flutter/physics.dart`.

Fixes #2441

These uses cases are now address by http.dart via http.readDataPipe.

* Fixed TimePicker crash and added tests

* Fixed TimePicker crash and added tests

* fixed a Lint-O

* move the material_gallery/demos.dart script

* rename all_demos.dart to all.dart

* add import guidance to library-level docs

* add import docs to other packages

* fix review comment

* clarify which libraries aren't meant to be directly imported