This patch updates the date picker to match the new spec.

We're still missing a fade effect when scrolling from one month to
another and we're missing the landscape layout.

Fixes #3558

Also, clean up the DatePicker to use more modern technology, such as a
grid for displaying the days of the month.

Fixes #3976

Also, make some previously private classes public for better
documentation.

* ScrollableBuilder tweaks

Also, add some missing docs to http.dart and widgets.dart.

* Add a Scrollable builder, refactor ScrollableList, et al

* Add space between the dialog demo buttons

* removed vestigial code

This reverts commit 55f9145e.

Turns out that this commit breaks apps that use the material library,
because of the _errorTextStyle DefaultTextStyle which has inherit:true.
Just setting it to false doesn't work, unfortunately, because then you
hit some sort of issue with merging that text style with others that
have inherit:true.

To make it clear that this constructor requires an explicit style. Also
throw a descriptive error recommending the inherit constructor for
styles with the inherit bit set.

Fixes #3842

* send debug events for flutter lifecycle events

* remove the route event

- Handles appear with tap or long press.
- Toolbar appears with long press on text, or tap on handle.
- Correct toolbar items shown depending on context.

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 context and child arguments are required.  We should assert that
they're non-null.

Fixes #3843

* Use mojo clipboard service for copy/paste toolbar.

BUG=https://github.com/flutter/flutter/issues/1567

Fixes #417

TextAlign applies to a whole paragraph instead of applying to an individual
text span. This patch moves the property out of TextStyle and into a separate
property on Text and RichText.

It has no more clients.

Follow-up to #3734.

Fixes #2484

Fixes #3747

* First pass at a material-style copy/paste toolbar.

This mimics the toolbar you see when selecting text in an Android
material app. There's still more to do (like integration with the system
clipboard), but this seemed like a good stopping point.

BUG=https://github.com/flutter/flutter/issues/1567

Turns out we were storing the same object in the base class and the
subclass. For pretty much every AnimatedWidget subclass.

* ClipPath

* Add a test for ClipOval and ClipPath

* Track the worst frame in perf tests

* Use backticks in "/// Returns `null` if"

* LayoutBuilder Widget

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

Fixes https://github.com/flutter/flutter/issues/3493

 - rebuild stateless widgets that have dependencies when their ancestors change but they don't

Fixes https://github.com/flutter/flutter/issues/3120

 - rebuild widgets that tried to inherit from a widget that didn't exist, when the widget is added

This adds a pointer and a bool to Element, which isn't great. It also adds a more or less complete tree walk when you add a new Inherited widget at the top of your tree, which isn't cheap.

Instead of incorporating the margin into the button, introduce a ButtonBar
widget that supplies the proper spacing between the buttons. Also, make these
buttons more configurable via ButtonTheme so that dialogs can change the
minWidth and padding of the buttons as required by the spec.

Fixes #1843
Fixes #3184

Also add dartdocs.

* 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

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

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.