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.

Don't send usage information on Chrome infra bots

We now describe what each parameter does.

Fixes #4256

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

This required refactoring some cases where we weren't following the rules for
the protected annotation.

Although beautiful, having a continuous animation on the front page of the
gallery gives the impression that Flutter burns battery and makes phones hot.
This patch replaces the animation with solid purple. However, we might
eventually want to use a static image.

This demo burns battery continuously even when there's no change to the visual
appearance. This patch removes this demo to avoid giving the impression that
Flutter burns battery.

Related to #4120

Bump Dart SDK to 1.17.0-dev.6.1.

Fixes #4298

Latest RC.  Fixes `dartdoc`.

Previously the floating action button would disappear during transitions. Now
we animate properly. I've also centralized the animation in Scaffold instead of
spreading it across the Scaffold and the FloatingActionButton.

Fixes #1718

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 a race condition with isolate notification

* return a future

* Fix fallout from intrinsics changes

* Move away from isInfinite in favor of isFinite

This patch updates how we generate xcconfig files to match what
xcode_backend.sh expects.

This reverts commit 215e1b89.

It broke the Mac build: https://build.chromium.org/p/client.flutter/builders/Mac/builds/1097

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.

As requested by the material design team.

Fixes #4182

Currently this service just returns mock data, but it exercises the plumbing.

Update analyzer dep to 0.27.4-alpha.9