Change Mac OS to OS X in dart:io dartdoc

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 711793a007eba55fce6e56b128912460992809bb..8a493a226a26d3401ff2c1023d2c3e83a9e263bb 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -729,7 +729,7 @@ abstract class RandomAccessFile {
    * *NOTE* file locking does have slight differences in behavior across
    * platforms:
    *
-   * On Linux and Mac OS this uses advisory locks, which have the
+   * On Linux and OS X this uses advisory locks, which have the
    * surprising semantics that all locks associated with a given file
    * are removed when *any* file descriptor for that file is closed by
    * the process. Note that this does not actually lock the file for
@@ -762,7 +762,7 @@ abstract class RandomAccessFile {
    * *NOTE* file locking does have slight differences in behavior across
    * platforms:
    *
-   * On Linux and Mac OS this uses advisory locks, which have the
+   * On Linux and OS X this uses advisory locks, which have the
    * surprising semantics that all locks associated with a given file
    * are removed when *any* file descriptor for that file is closed by
    * the process. Note that this does not actually lock the file for