Make Platform abstract

Bug:#52444
Change-Id: Ia4f98162f2b60d23c27ed9d8f78f70d308785650
CoreLibraryReviewExempt: landing previously-approved change
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/379740
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>

Author: brianquinlan

CHANGELOG.md

@@ -12,6 +12,13 @@
 
 ### Libraries
 
+#### `dart:io`
+
+- **Breaking Change** [#52444][]: Removed the `Platform()` constructor, which
+  has been deprecated since Dart 3.1.
+
+[#52444]: https://github.com/dart-lang/sdk/issues/52444
+
 #### `dart:js_interop`
 
 - Added constructors for `JSArrayBuffer`, `JSDataView`, and concrete typed array

sdk/lib/io/platform.dart

@@ -58,7 +58,7 @@ part of dart.io;
 ///   }
 /// }
 /// ```
-final class Platform {
+abstract final class Platform {
   /// The number of individual execution units of the machine.
   static final numberOfProcessors = _Platform.numberOfProcessors;
 
@@ -243,8 +243,4 @@ final class Platform {
   /// `"\r\n"`
   @pragma("vm:platform-const")
   static String get lineTerminator => isWindows ? '\r\n' : '\n';
-
-  @Deprecated("Do not instantiate this class, it will become abstract in a "
-      "future Dart version")
-  Platform();
 }