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

tests/standalone/io/options_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Stress test isolate generation.
 // DartOptions=tests/standalone/io/options_test.dart 10 options_test 20
 
 main() {
   var opts = new Options();
 
   // Basic test for functionality.
   Expect.equals(3, opts.arguments.length);
   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'));
 
   // Now add an additional argument.
   opts.arguments.add("Fourth");
   Expect.equals(4, opts.arguments.length);
   Expect.equals(10, Math.parseInt(opts.arguments[0]));
   Expect.equals("options_test", opts.arguments[1]);
   Expect.equals(20, Math.parseInt(opts.arguments[2]));
   Expect.equals("Fourth", opts.arguments[3]);
 
   // Check that a new options object still gets the original arguments.
   var opts2 = new Options();
   Expect.equals(3, opts2.arguments.length);
   Expect.equals(10, Math.parseInt(opts2.arguments[0]));
   Expect.equals("options_test", opts2.arguments[1]);
   Expect.equals(20, Math.parseInt(opts2.arguments[2]));
 }