Remove 'abstract' from 'class Platform'

Bug: #52138
Change-Id: Ibef54f5369d4b9374e9d58939ee26c853021bf74
CoreLibraryReviewExempt: Partial revert of 9c702bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/297160
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

Author: brianquinlan

sdk/lib/io/platform.dart

@@ -58,7 +58,7 @@ part of dart.io;
 ///   }
 /// }
 /// ```
-abstract final class Platform {
+final class Platform {
   static final _numberOfProcessors = _Platform.numberOfProcessors;
   @pragma("vm:platform-const")
   static final _pathSeparator = _Platform.pathSeparator;