Deprecate the Platform constructor

Bug:https://github.com/dart-lang/sdk/issues/52138
Change-Id: I9137ffe7caa1a1e9441d48b030342a528d99e8f6
CoreLibraryReviewExempt: deprecation, aske on holiday
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/303086
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

CHANGELOG.md

@@ -15,6 +15,7 @@
 - **Breaking change** [#51486][]:
   - Added `sameSite` to the `Cookie` class.
   - Added class `SameSite`.
+- Added a deprecation warning when `Platform` is instantiated.
 - Added `Platform.lineTerminator` which exposes the character or characters
   that the operating system uses to separate lines of text, e.g.,
   `"\r\n"` on Windows.

sdk/lib/io/platform.dart

@@ -252,4 +252,8 @@ 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();
 }