* rename Element.visitChildrenForSemantics to debugVisitOnstageChildren

* address comments

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* Revert "Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)"

This reverts commit ceb814aa.

Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)

This reverts commit d46e208b.

* Change all instances of '$runtimeType#$hashCode' to use the describeIdentity method.

The describeIdentity method generates a shorter description with a consistent length
consisting of the runtime type and the a 5 hex character long truncated
version of the hash code.

* preview of prefer_asserts_in_initializer_list lint

* fix issue

Trivial doc typo.

The enum is called StackFit, so the property name "fit" makes more sense.

Also:

 * Add three explicit sizing modes to Stack for non-positioned
   children: loose, expand, and passthrough. (All three are used.)

 * Fix a bug whereby layers would try to paint in the same frame as
   they were removed from layout (but not detached).

 * Fix a bug whereby Offstage wasn't properly marking the parent dirty
   when changing its sizedByParent flag.

 * Explicitly make Overlay expand non-positioned children.

 * Explicitly have InputDecoration pass through the constraints from
   its Row to its Stack children.

* more widget const constructors

* prefer const constructors

* address review comments

We now attempt to explain what went wrong.

Fixes #9509

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

* add @required when there's an assert not null

* address review comments

Covers lib/ in package:flutter.

...to avoid confusion with "detachRenderObject" which is basically
unrelated.

Closes https://github.com/flutter/flutter/issues/6141

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.

See #4434.

Fixes https://github.com/flutter/flutter/issues/5840

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.

Also, add more dartdoc to widgets.dart.

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

For consistency.

Fixes #4142

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 #3197

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308