Add executable to Options interface to get the name of the executable used to execute the script.

tests/standalone/io/options_test.dart

Index: tests/standalone/io/options_test.dart
diff --git a/tests/standalone/io/options_test.dart b/tests/standalone/io/options_test.dart
index 084d38c4df0e9e9f369282e8926abb2d1ca62ac1..f9175aaa30cd4d7ed2246ffaff157994e83788e5 100644
--- a/tests/standalone/io/options_test.dart
+++ b/tests/standalone/io/options_test.dart
@@ -12,6 +12,7 @@ main() {
   Expect.equals(10, Math.parseInt(opts.arguments[0]));
   Expect.equals("options_test", opts.arguments[1]);
   Expect.equals(20, Math.parseInt(opts.arguments[2]));
+  Expect.isTrue(opts.executable.contains('dart'));
   Expect.isTrue(opts.script.replaceAll('\\', '/').
                 endsWith('tests/standalone/io/options_test.dart'));