VM deadlocks when passed the "--packages" flag

When the VM is passed the `--packages` flag pointing to a valid package spec, it deadlocks without ever starting the application. For example, if you have:

``` dart
// app.dart
void main() => print("hello!");
```

```
// .packages
myapp:lib/
```

And you run `dart --packages=.packages app.dart`, it will never print anything and never exit.
