* 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

* Adds DropDownFormField Convenience Class

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

If `items` or `onChanged` is null the button will be disabled, the down arrow will be grayed out, and
the new `disabledHint` will be shown (if provided).

* Adds comments clarifying the procedure used to render the menu as well as tests verifying various dropdown menu button positioning and initial scroll states.

In long lists this resulted in the dropdown scrolling to the very last
item in its list. Now clamping the value at `0.0`. Added a test to
verify that the selected item aligns with the button to test the offset.

Fixes flutter/flutter#15346

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Fix Dropdown where icon not set to end of button

* Expand DropdownButton based on isExpanded flag

* Add dropdown golden tests

Fixes #13135, #16606

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.

The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.

* spelling: accommodate

* spelling: allotted

* spelling: anonymous

* spelling: artificial

* spelling: associated

* spelling: asset

* spelling: button

* spelling: canvas

* spelling: compatibility

* spelling: coverage

* spelling: condition

* spelling: decoration

* spelling: deferring

* spelling: diameter

* spelling: direction

* spelling: displacement

* spelling: dropdown

* spelling: needing

* spelling: environment

* spelling: exited

* spelling: expansion

* spelling: explore

* spelling: families

* spelling: horizontal

* spelling: increment

* spelling: indices

* spelling: internationalization

* spelling: labrador

* spelling: localizations

* spelling: midflight

* spelling: milliseconds

* spelling: minimum

* spelling: multiple

* spelling: multiplication

* spelling: navigator

* spelling: overridden

* spelling: package

* spelling: performance

* spelling: platform

* spelling: porsche

* spelling: position

* spelling: preceded

* spelling: precede

* spelling: precedence

* spelling: print

* spelling: property

* spelling: readily

* spelling: reproducibility

* spelling: rounded

* spelling: scroll

* spelling: separate

* spelling: separator

* spelling: services

* spelling: specific

* spelling: specify

* spelling: synchronously

* spelling: through

* spelling: timeout

* spelling: triangle

* spelling: trivial

* spelling: unusual

* spelling: then

* spelling: vertically

* spelling: visible

* spelling: visited

* spelling: voice

* Use explicit types for onClick handler in diagnostics_test.

This test formats it to string and expects exact match.

Before this change onClick had type () => dynamic in Dart 1 and () => Null in
Dart 2.

* Fix strong mode issues in packages/flutter/test/material/dropdown_test.dart

Pass explicit type argument to renderObjectList(...) in expressions like

     List<RenderBox> l = tester.renderObjectList(...).toList();

Dart 2 mode inference is not strong enough to infer RenderBox as a type argument
fto renderObjectList and passes RenderObject instead, which later leads to
runtime check failures (because List<RenderObject> is not List<RenderBox>).

* Fix strong mode issues in packages/flutter/test/material/theme_test.dart.

Pass type argument explicity to widgetList. Dart 2 type inference can't infer
it by itself.

* Fix strong mode issue packages/flutter/test/widgets/unique_widget_test.dart

Pass correct type argument to GlobalKey.

* Fix type annotation in packages/flutter/test/material/app_test.dart.

pushNamed returns Future<Object> not Future<String>.

This fixes DropdownButtons to align their popups correctly in RTL as well.

Also while I was there I fixed the issue with text scale factor in the gallery.

Previously, ExpansionPanel would do weird things if interacted with
when it was already animating. This is fixed and there's now a test.

Also:

 * Minor fixes to make the gallery work in RTL, not that there's
   any way to see that without hard-coding the framework to RTL.
   But at least I'll be less annoyed when doing that.

 * Some trivial code and documentation cleanup.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Fix dropdown buttons menus being clipped by the edge of the screen

Notice that the default alignment for Stack needs to know the
directionality.