Also, add more dartdoc to widgets.dart.

This patch starts working through the missing dartdocs in widgets.dart.

Previously we supplied individual parameters to the various drag and pan
callbacks. However, that approach isn't extensible because each new
parameter is a breaking change to the API.

This patch makes a one-time breaking change to the API to provide a
"details" object that we can extend over time as we need to expose more
information. The first planned extension is adding enough information to
accurately produce an overscroll glow on Android.

The old code had gotten crufty with all the refactors. This simplifies
it down to what it really means.

Also, add a bunch of tests.

We used to rely on BoxConstraints.deflate's clamping, but now we have to
roll our own.

All of the public APIs in rendering.dart now have dartdocs.

This introduces the key parts of a paginated data table, not including
the built-in pagination features.

* Provide more data for the data table demo, so there's data to page.

* Introduce a ChangeNotifier class which abstracts out
  addListener/removeListener/notifyListeners. We might be able to use
  this to simplify existing classes as well, though this patch doesn't
  do that.

* Introduce DataTableSource, a delegate for getting data for data
  tables. This will also be used by ScrollingDataTable in due course.

* Introduce PaginatedDataTable, a widget that wraps DataTable and only
  shows N rows at a time, fed by a DataTableSource.

...and fix some grid and table intrinsic issues it uncovered.

* Fix fallout from intrinsics changes

* Move away from isInfinite in favor of isFinite

There are many more places we can use this annotation. This patch starts out
small to test the waters.

This patch starts using the mustCallSuper annotation for several of the key
lifecycle callbacks in the framework. I haven't added it to didUpdateConfig
because there are a large number of non-compliant overrides. We should discuss
whether we want to use it there.

I pulled the trigger to land the previous patch too quickly and missed a
number of comments.

We used to use this mechanism to paint scrollbars, but it's awkward. The
new approach to scrollbars is much cleaner. This mechanism has no other
clients, so we should remove it.

The new mojom.dart code makes mocking services a bit tricky. I've filed
https://github.com/domokit/mojo/issues/786 about improving that.

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

We now explain what coordinate system each argument is expected to be in.

Fixes #4205

The new version has an API change. Also, up our SDK requirements to be a
bit more realistic.

For consistency.

Fixes #4142

According to the profile for the flow manual test, we're spending the vast
majority of our time recording timeline traces. This patch removes the timeline
traces, which greatly improves performance.

Also, optimize TransformLayer to avoid one matrix memcpy. I filed
https://github.com/google/vector_math.dart/issues/166 about an API that would
make this even faster.

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.

Use the FlutterError hotness instead.

Fixes #1290

* Added performance instrumentation for compositing addToScene() calls.

* Switched to non-callback versions of the Timeline API.

* Fixed Dart formatting nit.

- 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`.

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.

This patch sizes the menu such that it is always on screen, but doesn't scroll
the menu to ensure that the currently selected item is always visible and on
top of the button. That will need to wait for a later patch.

Also, teach CustomPaint how to repaint animations more efficiently.

Fixes #3720

The `flutter` package gets more types!

Follow-up from: https://github.com/flutter/flutter/pull/3727.

* ClipPath

* Add a test for ClipOval and ClipPath

* Track the worst frame in perf tests

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

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

If there's a max height or width, we should factor that into the intrinsic
sizing for the other dimension.

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