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..455e5568906e659bce3323de2e5326b35a0c5884 100644
--- a/runtime/bin/directory.h
+++ b/runtime/bin/directory.h
@@ -10,6 +10,12 @@
 
 class Directory {
  public:
+  enum ExistsResult {
+    UNKNOWN,
+    EXISTS,
+    DOES_NOT_EXIST
+  };
+
   static void List(const char* path,
                    bool recursive,
                    Dart_Port dir_port,
@@ -17,6 +23,12 @@ class Directory {
                    Dart_Port done_port,
                    Dart_Port error_port);
 
+  static ExistsResult Exists(const char* path);
+
+  static bool Create(const char* path);
+
+  static bool Delete(const char* path);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Directory);
 };