Stop using Match.group.

The `Match.operator[]` does the same thing and is
generally recommended (and shorter).
(I want to deprecate `group` and `groups`)

Tested: Refactoring.
CoreLibraryReviewExempt: Calling equivalent function.
Change-Id: I4c758968ae622fe16b7322be1b29b05b91e7fcd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/489021
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>

third_party/pkg/dap/tool/codegen.dart

@@ -38,8 +38,7 @@ class CodeGenerator {
         name
             .replaceAll(RegExp(r'^_+'), '')
             // Also replace any other underscores to make camelCase
-            .replaceAllMapped(
-                RegExp(r'_(.)'), (m) => m.group(1)!.toUpperCase());
+            .replaceAllMapped(RegExp(r'_(.)'), (m) => m[1]!.toUpperCase());
   }
 
   /// Re-wraps [lines] at [maxLength] to help keep comments for indented code