This pull request reverts the changes in #155757.
The auto-formatter created conflicts in the master branch, so the revert was performed manually in #160643.
For this cherry-pick PR, I was able to run `git revert 21381d843f3feb32640ae385aedd8e5c4003696b` without any problems.
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### Issue Links
bug reports: #160196, #160555
cherry-pick request: #161176
### Target
stable
### Changelog Description
Passing a list literal to a `DropdownMenu` causes the widget to reset to the `initialSelection` after each build.
### Impacted Users
This affects anyone using the [DropdownMenu](https://api.flutter.dev/flutter/material/DropdownMenu-class.html) widget.
### Impact Description
The impact usually consists of the text value being inconveniently reset each time the widget is rebuilt. (In some cases it can be a fatal crash: the code sample from #160196 shows how this change can lead to an infinite build loop.)
### Workaround
This regression can be mitigated by caching & modifying a single list instance, rather than using a list literal for the `DropdownMenu` constructor.
### Risk
low
### Test Coverage
yes
### Validation Steps
#160643 added a regression test for this revert.
The fix can also be verified by running the code sample from #160196 and verifying that there is no infinite build loop.
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