Add a Platform.lineTerminator static method

Bug: https://github.com/dart-lang/sdk/issues/52379
Change-Id: Ic3a7f06252f8a69dcfdb29c00f16557c34529652
CoreLibraryReviewExempt: Aske is on holiday
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/297260
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

CHANGELOG.md

@@ -15,6 +15,9 @@
 - **Breaking change** [#51486][]:
   - Added `sameSite` to the `Cookie` class.
   - Added class `SameSite`.
+- 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.
 
 [#51486]: https://github.com/dart-lang/sdk/issues/51486
 

sdk/lib/io/platform.dart

@@ -240,4 +240,16 @@ final class Platform {
   /// possibly followed by whitespace and other version and
   /// build details.
   static String get version => _version;
+
+  /// The current operating system's default line terminator.
+  ///
+  /// The default character sequence that the operating system
+  /// uses to separate or terminate text lines.
+  ///
+  /// The line terminator is currently the single line-feed character,
+  /// U+000A or `"\n"`, on all supported operating systems except Windows,
+  /// which uses the carriage-return + line-feed sequence, U+000D U+000A or
+  /// `"\r\n"`
+  @pragma("vm:platform-const")
+  static String get lineTerminator => isWindows ? '\r\n' : '\n';
 }

tests/standalone/io/platform_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:convert';
 import "dart:io";
 import "dart:isolate";
 
@@ -31,6 +32,10 @@ test() {
   Expect.equals(Platform.isAndroid, Platform.operatingSystem == "android");
   var sep = Platform.pathSeparator;
   Expect.isTrue(sep == '/' || (os == 'windows' && sep == '\\'));
+  final eol = Platform.lineTerminator;
+  Expect.isTrue(
+      (eol == '\n' && os != 'windows') || (eol == '\r\n' && os == 'windows'),
+      "unexpected line ending ${utf8.encode(eol)} on $os");
   var hostname = Platform.localHostname;
   Expect.isTrue(hostname is String && hostname != "");
   var environment = Platform.environment;

tests/standalone_2/io/platform_test.dart

@@ -4,6 +4,7 @@
 
 // @dart = 2.9
 
+import 'dart:convert';
 import "dart:io";
 import "dart:isolate";
 
@@ -33,6 +34,10 @@ test() {
   Expect.equals(Platform.isAndroid, Platform.operatingSystem == "android");
   var sep = Platform.pathSeparator;
   Expect.isTrue(sep == '/' || (os == 'windows' && sep == '\\'));
+  final eol = Platform.lineTerminator;
+  Expect.isTrue(
+      (eol == '\n' && os != 'windows') || (eol == '\r\n' && os == 'windows'),
+      "unexpected line ending ${utf8.encode(eol)} on $os");
   var hostname = Platform.localHostname;
   Expect.isTrue(hostname is String && hostname != "");
   var environment = Platform.environment;