Reapply directory error handling change.

runtime/bin/directory_impl.dart

Index: runtime/bin/directory_impl.dart
diff --git a/runtime/bin/directory_impl.dart b/runtime/bin/directory_impl.dart
index 4e962e41e1e9f50d5f623f02262120a82f6d2ad7..485dfadfcac5e6b83b22824b6f9ee4ff70552399 100644
--- a/runtime/bin/directory_impl.dart
+++ b/runtime/bin/directory_impl.dart
@@ -36,7 +36,6 @@ class _Directory implements Directory {
   bool existsSync() {
     int exists = _exists(_path);
     if (exists < 0) {
-      // TODO(ager): Supply a better error message.
       throw new DirectoryException("Diretory exists test failed: $_path");
     }
     return (exists == 1);
@@ -44,14 +43,12 @@ class _Directory implements Directory {
 
   void createSync() {
     if (!_create(_path)) {
-      // TODO(ager): Supply a better error message.
       throw new DirectoryException("Directory creation failed: $_path");
     }
   }
 
   void deleteSync() {
     if (!_delete(_path)) {
-      // TODO(ager): Supply a better error message.
       throw new DirectoryException("Directory deletion failed: $_path");
     }
   }