#2242. Add line endings tests (#2250)

.gitattributes

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+# Tests below shouldn't be part end-of-line conversion upon checkin or checkout
+Language/Reference/Lexical_Rules/line_endings_cr_t01.dart -text
+Language/Reference/Lexical_Rules/line_endings_crlf_t01.dart text eol=crlf

Language/Reference/Lexical_Rules/line_endings_cr_t01.dart

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+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file// 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./// @assertion Dart source text is represented as a sequence of Unicode code/// points.////// @description Checks that Dart program may use CR line endings/// @author sgrekhov22@gmail.comString hello() => "Hello";class World {  String world() {    return "World!";  }}main() {  print(hello() + " " + World().world());}

Language/Reference/Lexical_Rules/line_endings_crlf_t01.dart

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+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+/// @assertion Dart source text is represented as a sequence of Unicode code
+/// points.
+///
+/// @description Checks that Dart program may use CRLF line endings
+/// @author sgrekhov22@gmail.com
+
+String hello() => "Hello";
+
+class World {
+  String world() {
+    return "World!";
+  }
+}
+
+main() {
+  print(hello() + " " + World().world());
+}