This adds a "three point" cubic curve that combines two cubic curves into one curve, where the control points are all in the same coordinate system. It's called "three point" because it incorporates a middle point that the curve runs through, and that point is in addition to the existing implicit (0,0) and (1,1) points that a Curve already has.

I also added an example of this curve, which is just a more emphasized version of the easeInOutCubic.

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

[flutter] throw more specific error messages if a lerp'd type does not conform to Tween API (#74684)

Fix the duration assertion error for the _animateToInternal method and refine the error description (#71137)

* enable null_check_on_nullable_type_parameter and tighten_type_of_initializing_formals

* fix CI

* Add punctuation for unterminated sentences in doc comments

* Addressing review nit

* migrate animation to nullsafety

* nullable tween

* fix generic type issue

* address review comment

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This PR is a follow up of https://github.com/flutter/flutter/pull/42640
Some changes of that PR includes redundant changes using FlutterError.fromParts constructor even though it's not necessary.

Some minor changes are:

- Remove one unnecessary todo
- Fix indent consistencies

This PR modifies the existing API docs samples to use DartPad so that all of the samples are now interactive apps on the API docs site.

It also removes the restriction for the max width of the description area so that the dartpad region can expand horizontally.

I updated the first paragraph on the API docs to indicate that Flutter is more than just mobile now (same text as the README.md at the top level).

I modified a few of the examples so that they looked nicer, and fit better on the page.

I added the sample description text above each DartPad instance, since that often defines the context of the example.

I removed animations and images when they were redundant with the sample content. There were a few that made sense to keep, so I did.