Add dart2js batch mode support

Add dart2js batch mode support

This removes an old map from runtimes to executables that is no longer needed and instead assumes a ProcessCommand that has an executable.
The added benefit of this is that it works with --use-sdk (and there are less code to maintain)

The flag to enable dart2js batch mode is temporary until we can switch the default.

When we can, this can go away and we can reuse the noBatch flag (which defaults to false)

R=whesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=36628

[Bill Hesse, 2014-05-26 10:50:59]

lgtm

[ricow1, 2014-05-26 10:52:46]

+kevin
When we start defaulting to this dart2dart will also be run in batch mode, but
that should only matter compared to the flags we pass in on stdin as far as I
can see. We should validate that it is working as expected though

[ricow1, 2014-05-26 11:03:07]

Committed patchset #1 manually as r36628 (presubmit successful).