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## Description The current behavior of Flutter regarding `DropdownMenu` text field is very basic: `DropdownMenu` updates the text field once a selection has been made with its coordinating `label`, and nothing more. If the selection's `label` changes, the text field will still show the old value, although the menu buttons have been updated. This not only results in the need to write a lot of boilerplate code to match the label to the current selection, but it's also counterintuitive. The `DropdownMenu` should handle rematching by itself. Issue #155660, touched on this, and a solution was introduced in #155757. It was later reverted due to it restricting some use cases. That issue & solution, however, only address `initialSelection`. If another selection was made and its corresponding `label` was changed, the text field would still show the old value, bringing us back to square one. This PR should not conflict with any previous behavior of `DropdownMenu`: any new value provided by `initialSelection` or via controller would not be overwritten by rematching. However, entries with the same value will be mapped to a single label (the first matched entry's label). ## Related Issues - Fixes #155660. ## Tests Added 3 tests. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing.