Add support for directory renaming to dart:io.

runtime/bin/directory_posix.cc

Index: runtime/bin/directory_posix.cc
diff --git a/runtime/bin/directory_posix.cc b/runtime/bin/directory_posix.cc
index 1c672a804d7e86ad6a6f0e51b2e20848f50a3a4c..7adb80f9156ab2d308131aee2181243b35b85a27 100644
--- a/runtime/bin/directory_posix.cc
+++ b/runtime/bin/directory_posix.cc
@@ -423,3 +423,10 @@ bool Directory::Delete(const char* dir_name, bool recursive) {
     return DeleteRecursively(dir_name);
   }
 }
+
+
+bool Directory::Rename(const char* path, const char* new_path) {
+  ExistsResult exists = Exists(path);
+  if (exists != EXISTS) return false;
+  return (TEMP_FAILURE_RETRY(rename(path, new_path)) == 0);
+}