* fix indentof statements in BlockFunctionBody

* fix indentof statements in BlockFunctionBody in tests

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

This reverts commit 1d93e924.

In particular, make debugVisitOnstageChildren only work in debug mode, and add documentation.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

when a hot reload is done after modifying a component in the flutter
package.

* roll engine version in flutter/flutter to b05efc204090f430387398d3c35589571a2fe16b

* Fix analyzer warnings.

* 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