SDK: Deprecate the ability to implement RegExp and RegExpMatch.

Work towards https://github.com/dart-lang/sdk/issues/55608

Change-Id: Ib686c5a070c54907e9f4d900cd0736ca70a712d0
CoreLibraryReviewExempt: No behavioral change
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/453460
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

CHANGELOG.md

@@ -149,6 +149,8 @@ instead.
     deprecated.
   - [`@Deprecated.instantiate()`][] indicates the ability to instantiate a
     class is deprecated.
+- The ability to implement the RegExp class and the RegExpMatch class is
+  deprecated.
 
 [#61392]: https://dartbug.com/61392
 [`@Deprecated.extend()`]: https://api.dart.dev/dev/latest/dart-core/Deprecated/Deprecated.extend.html

sdk/lib/core/regexp.dart

@@ -213,6 +213,10 @@ part of "dart:core";
 /// when the regexp does not find a match.
 /// Several guides to [improving the performance of regular expressions](https://www.google.com/search?q=performance+of+regular+expressions)
 /// exist on the internet. Use these as inspirations, too.
+@Deprecated.implement(
+  "This class will become 'final' in a future release. "
+  "'Pattern' may be a more appropriate interface to implement.",
+)
 abstract interface class RegExp implements Pattern {
   /// Constructs a regular expression.
   ///
@@ -468,6 +472,10 @@ abstract interface class RegExp implements Pattern {
 ///   // This is the second message.
 /// }
 /// ```
+@Deprecated.implement(
+  "This class will become 'final' in a future release."
+  "'Match' may be a more appropriate interface to implement.",
+)
 abstract interface class RegExpMatch implements Match {
   /// The string captured by the named capture group [name].
   ///