Adds the CupertinoContextMenu widget for iOS 13 support.

Updates documentation and non-public API to use American spellings for
consistency with the rest of the codebase.

No changes to behaviour... other than how it's spelt.

* 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

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* Changes Cupertino page transition curves to make paging animations more similar to those of native iOS

* nav fix and test

MaterialPageRoute transitions are now defined by the Theme. Added (optional) support for Android P style page transitions.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Fixes resulting from audit of issues links

I looked at every link to GitHub in our repo. For cases where we had a TODO that was waiting for a bug to be fixed, and the bug has now been fixed, I applied the pending change. For cases where the link was out of date, I updated the link.

* Update run_test.dart

skip this test again since it failed on linux and macos bots

Adding CupertinoActionSheet, showCupertinoModalPopup

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.

Semantics object support for edge triggered semantics