[dart:io] Adds functions to set file access and modification time

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index 2a4a1f5140fd857e4b37bd0b932e4dc7776cc106..fe4777e5cb4a068dd9f1f3dd79c05cb12f357075 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -359,7 +359,7 @@ abstract class FileSystemEntity {
    * with .type set to
    * FileSystemEntityType.NOT_FOUND and the other fields invalid.
    */
-  Future<FileStat> stat();
+  Future<FileStat> stat() => FileStat.stat(path);
 
   /**
    * Synchronously calls the operating system's stat() function on the
@@ -371,7 +371,7 @@ abstract class FileSystemEntity {
    * If the call fails, returns a [FileStat] object with .type set to
    * FileSystemEntityType.NOT_FOUND and the other fields invalid.
    */
-  FileStat statSync();
+  FileStat statSync() => FileStat.statSync(path);
 
   /**
    * Deletes this [FileSystemEntity].