Make `future` and `stream` required arguments in their respective builder widgets (#125838)

cc'ing existing conversation participants: @domesticmouse @srawlins
cc'ing to request review: @goderbauer 

This PR makes the following constructor arguments required:
1. `FutureBuilder.future`
2. ``
3. ``

This fixes: (dupe of 83081)

This obviates:
(I suggest we skip straight to merging this PR as this should be a low impact breaking change-assuming few to no devs are intentionally using the builders without their relevant arguments, however we could always merge 4309 first and then this) 
(The above PR required that at least one of future and initial data be non-null, this is undesirable as there are plenty of valid reasons to have both arguments be null)

See above issues for a deeper dive, but here is a summary:
It is very easy for a developer to forget to specify `future` or `stream` when using the respective `*Builder` widgets. This produces a non-obvious failure where the UI sits in a "no data yet received" state. It is easy for a dev to misinterpret this as the async work backing the future/stream hanging and they thus waste a lot of time trying to debug the async work.
As such, we should require these two constructor arguments to make it impossible/much harder for devs to make this time-wasting mistake.

This is a breaking change. However, it should break only a small number of active projects given that using a builder without specifying `future` or `stream` seems highly niche.
The only place I've found non-accidental examples of this is in widget tests where you're calling `pumpWidget` with and without these arguments to test `*Builder.didUpdateWidget`'s behavior. In this and similar cases, it is a trivial fix to add `future: null`/`stream: null`.

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
2 files changed
tree: 05343120c7f0fae80d8a638a5f39a9debde81068
  1. .github/
  2. .vscode/
  3. bin/
  4. dev/
  5. examples/
  6. packages/
  7. .ci.yaml
  8. .cirrus.yml
  9. .gitattributes
  10. .gitignore
  11. analysis_options.yaml
  16. dartdoc_options.yaml
  17. flutter_console.bat

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