support for file I/O

[danrubel]

For Frog to run on the VM, we need a VM dart:io library to support file I/O

[DartBot]

This comment was originally written by jimhug@google.com

---

Frog does run on the VM today. It uses the file_system_vm.dart class to do
this which uses the VMs file I/O libraries.

However, this compiler needs to use a Python stub (frog.py) to handle
command-line arguments, running subprocesses and setting the process exit
code correctly. I believe that those last three features are the key ones
to get added to the VM to best support frog.

---

Added Triaged label.

[danrubel]

Noted and opened separate issues:
http://code.google.com/p/dart/issues/detail?id=309
http://code.google.com/p/dart/issues/detail?id=310

---

Added Done label.