Generate dill files for faster startup.

[bsutton]

When dcli launches a script it is rather slow.
If we compiled the script to a dill file on first run then it would run much faster after that.
Only issue is that we need to be able to efficiently detect if a package src changes so we know when to re-compile the dill.
Also need to recompile if the SDK version changes.

dart-lang/pub#1683

[bsutton]

doesn't look like this will be possible.