Clarify the compile error for "same" map keys. (#2982)

* Clarify the compile error for "same" map keys.

This shouldn't affect the implemented behavior but it avoids an unclear
specification corner where two record map keys could or could not be
considered the same since record identity is deliberately unspecified.

Fix #2979.

* Link to record proposal's existing definition of structural equivalence.

Author: munificent

accepted/future-releases/0546-patterns/feature-specification.md

@@ -621,20 +621,15 @@ It is a compile-time error if:
     expressions in a future release without it being a breaking change, similar
     to default values in parameter lists.*
 
-*   Any two keys in the map are identical. *Duplicate keys are likely to be a
-    copy/paste error. If you want to match the value associated with some key
-    against multiple patterns, you can always use an `||` pattern.*s
-
-*   Any two record keys which both have primitive equality are equal. *Since
-    records don't have defined identity, we can't use the previous rule to
-    detect identical records. But records do support an equality test known at
-    compile time if all of their fields do, so we use that.*
+*   Any two keys in the map are [structurally equivalent][]. *Duplicate
+    keys are likely to be a copy/paste error. If you want to match the value
+    associated with some key against multiple patterns, you can always use an
+    `||` pattern.*s
 
 *Note that `mapPatternEntries` is not optional, which means it is an error for
 a map pattern to be empty.*
 
-*Note that we don't require map keys to have primitive equality, to enable
-more flexibility in key types.*
+[structurally equivalent]: https://github.com/dart-lang/language/blob/master/accepted/future-releases/records/records-feature-specification.md#canonicalization
 
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