Add exists, create and delete to directory implementation on Linux and Mac.

runtime/bin/directory.h

Index: runtime/bin/directory.h
diff --git a/runtime/bin/directory.h b/runtime/bin/directory.h
index 0c6b7e23c4f1c2d9d6bb6260c423af12b6a0ea06..d384e1a2791f075048e87d4f70dd4e51e03807a0 100644
--- a/runtime/bin/directory.h
+++ b/runtime/bin/directory.h
@@ -17,6 +17,12 @@ class Directory {
                    Dart_Port done_port,
                    Dart_Port error_port);
 
+  static bool Exists(const char* path, bool* result);
+
+  static bool Create(const char* path);
+
+  static bool Delete(const char* path);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Directory);
 };