Add feature flags for android, ios, and fuchsia (on by default). After updating the g3 rollers, the fuchsia feature will be turned off by default. Creates a simpler base type of feature flags for g3 to extend.

Updates android, ios, fuchsia workflows to use feature flags check.
Removes concept of stable artifacts and checks on flutter version.

Fixes #58999
#52859
#12768

complete removal of all generate functionality

Fixes #61508

`showDialog()` and `showGeneralDialog()` were allowing transparent
colors but then triggering an assertion down the stack while building
the modal barrier. The assertion existed to keep from animating from
transparent to transparent, but there's no need for the assertion,
since we can just treat the transparent case as the same as the null
case -- no animation necessary.

This was testing that concurrent reloads were rejected. This is now allowed (as of #55376 (which has non-devicelab integration + unit tests).

Remove git dependencies from pub-cache

#61487

* Promtoe WidgetTester.ensureVisible

* modify according to tvolkert@

https://github.com/flutter/flutter/issues/61496

The function passes most of its arguments through to
`_DialogRoute`, but it wasn't providing the default
values that `_DialogRoute` was, causing it to override
the defaults will null values.

The Xcode build almost always takes longer than 2 seconds. With the
operation classified as a "fast operation", the Flutter tool will
warn the user that the build is "taking an unexpectedly long time".
This updates the operation to be a "slow operation", which will only
warn the user if it takes 2+ minutes.

Allow configuring the flutter_manifest to support a synthetic package, this is done through flutter: generate: true.

When running pub get, insert a flutter_gen entry into the packages if it does not already exist. This points to .dart_tool/flutter_gen, which can be updated to contain the generated intl sources (But doesn't currently)

Adds an integration test that verifies this code can be run and imported when enabled.

Part of #60914

* Fixing patch for coming breaking change in HttpClientRequest

* Add noSuchMethod() and comment out abort()

* Add ignore: override_on_non_overriding_member

Add document for mouse cursor mapping on Windows and Linux.

* migrate foundation to nullsafety

* address review comments

Enable null safety asserts for web debug mode. This induces runtime asserts at the boundaries between null safe and non-null safe libraries. Adds integration test that validates assertion error is thrown.

#61042