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

runtime/bin/directory_win.cc

Index: runtime/bin/directory_win.cc
diff --git a/runtime/bin/directory_win.cc b/runtime/bin/directory_win.cc
index 4e5086fec3318f0c4d9735321946909ad677fdd6..624f5781a98d143c66e2aa842703c0594fd88b59 100644
--- a/runtime/bin/directory_win.cc
+++ b/runtime/bin/directory_win.cc
@@ -155,6 +155,7 @@ static bool ListRecursively(const char* dir_name,
   return completed;
 }
 
+
 void Directory::List(const char* dir_name,
                      bool recursive,
                      Dart_Port dir_port,
@@ -172,3 +173,21 @@ void Directory::List(const char* dir_name,
     Dart_Post(done_port, value);
   }
 }
+
+
+bool Directory::Exists(const char* dir_name, bool* result) {
+  UNIMPLEMENTED();
+  return false;
+}
+
+
+bool Directory::Create(const char* dir_name) {
+  UNIMPLEMENTED();
+  return false;
+}
+
+
+bool Directory::Delete(const char* dir_name) {
+  UNIMPLEMENTED();
+  return false;
+}