Adds assist for basic Flutter Builder

[b-stime]

Fixes issues such as

• Flutter IDE Wish: wrap with a builder, a stateful builder, or a layout builder flutter/flutter-intellij#814
• Add a 'wrap with (Flutter) Builder refactor' #33957 .

Essentially, in Flutter, it's often useful to create a new context in the UI/widget tree as a place to begin a search towards the trunk of the tree for various state structures attached to lower levels of the tree. The basic Builder widget is the official means for creating such a context/entry-point.

The patches in this PR are essentially a copy-paste-tweak from two types of existing, similar assists. The main origin was taken from FlutterWrapStreamBuilder which wraps client code with a constructor for the StreamBuilder sub-class. The new assist, provided here, is a little more basic/general than the StreamBuilder so it was able to also adopt assertion and test code from some of the other assists for basic widgets.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[b-stime]

@googlebot I signed it!

[bwilkerson]

Thanks for the contribution! The review will happen at https://dart-review.googlesource.com/c/sdk/+/194941. You can continue to make changes to the PR (if any are needed) and push them to this site, and they will automatically be mirrored to Gerrit.