Fuchsia: Adds File::Copy()

runtime/bin/directory_fuchsia.cc

Index: runtime/bin/directory_fuchsia.cc
diff --git a/runtime/bin/directory_fuchsia.cc b/runtime/bin/directory_fuchsia.cc
index 5ba6544bc1b05c31a8c58dd4fd25e10d6b252b1c..9259bd019164403da02b95544835ac2cd9e01138 100644
--- a/runtime/bin/directory_fuchsia.cc
+++ b/runtime/bin/directory_fuchsia.cc
@@ -355,6 +355,8 @@ bool Directory::Delete(const char* dir_name, bool recursive) {
     }
     return NO_RETRY_EXPECTED(rmdir(dir_name)) == 0;
   } else {
+    // TODO(MG-416): After the issue is addressed, this can use the same code
+    // as on Linux, etc.
     UNIMPLEMENTED();
     return false;
   }