Specify using unnecessary_breaks in the coding style.

Change-Id: Ibedb2fcd0ad5fdb6bfc3dcc4b88a02c9a9d78152
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337282
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/doc/implementation/coding_style.md

@@ -21,10 +21,10 @@ This section documents the styles we have agreed to follow.
 
 ### Lint rules
 
-None.
-
 - prefer_single_quotes - We don't enforce this in each of our packages yet, but
   we aspire to.
+- unnecessary_breaks - All our packages require at least Dart `3.0.0`, so we
+  can do it.
 
 ### Formatting and sorting