Add details for non-promotion causes/workarounds

[kwalrath]

Details in dart-lang/sdk#44900.

#3148 partially addressed this issue by adding the following:

https://dart.dev/tools/non-promotion-reasons
https://dart.dev/go/non-promo-write
https://dart.dev/go/non-promo-property
https://dart.dev/go/non-promo-this

The current plan is to have one go link for each kind of non-promotion, with each go link redirecting to a section of this new page. See dart-lang/sdk#44900 (comment) for all 11 proposed go links.

Example from Lasse & mit

Note that BadEvent needs to use year! because year could have been changed by an overriding getter in a subclass. GoodEvent shadows year with a local variable (var year = this.year) so that the code can just use year.

[kwalrath]

I believe this is covered by https://dart.dev/guides/language/effective-dart/usage_migrated#consider-copying-a-nullable-field-to-a-local-variable-to-enable-type-promotion (which will be moved to https://dart.dev/guides/language/effective-dart/usage once null safety is in the stable SDK).

I wonder if this should also be in the FAQ, which is based on an internal FAQ.

/cc @ryanmcginnis @davidmorgan

[kwalrath]

I'm working on this and hope to have something for @stereotype441 to look at early next week. I/O permitting. :)