Use `curly_braces_in_flow_control_structures` for `foundation`, `gestures`, `painting`, `physics` (#104610)

* Use `curly_braces_in_flow_control_structures` for `foundation`

* Use `curly_braces_in_flow_control_structures` for `gestures`

* Use `curly_braces_in_flow_control_structures` for `painting`

* Use `curly_braces_in_flow_control_structures` for `physics`

* fix comments

* remove trailing space

* fix TODO style

* migrates lib/foundation and lib/widgets

* migrates lib/painting and lib/material

* migrates lib/cupertino, lib/gestures, lib/services, lib/semantics and lib/rendering

* fixes failing tests

* migrates test/

* improves hashing of lists

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

Update the alt text to match recommendations in https://html.spec.whatwg.org/multipage/images.html#alt (#74089)

Now that https://github.com/dart-lang/sdk/issues/41985 is fixed, the
analyzer detects dead code due to "unnecessary" null checks.  Since we
want to retain null checks in Flutter even when they appear
unnecessary (in order to preserve runtime behavior in weak mode), we
need to suppress these dead code hints.

Note that one assertion is removed by this PR (`heightFactor != null
|| (heightFactor == 1.0 && heightDelta == 0.0)`).  This is because
`heightFactor == 1.0 && heightDelta == 0.0` can only be true if
`heightFactor != null`, so this assertion is equivalent to simply
asserting that `heightFactor != null`, which is already asserted two
lines above.

* migrate painting to nullsafety

* address review comments

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This converts Diagnosticable to be a mixin instead of an abstract class, so that it can be used to add diagnostics to classes which already have a base class.

It leaves in place the DiagnosticableMixin mixin, since there are some plugins that are still using it, and removing it would mean that those plugins wouldn't work with master branch Flutter anymore. DiagnosticableMixin will be removed once this mixin version of Diagnosticable makes its way to the stable branch.