I've tried running dart2js both choosing "Run as JavaScript" from the context menu, and from Tools -> Generate JavaScript.
Both methods producing the same console output (an error):
Running dart2js...
No se esperaba \dart-sdk\bin\dart2js.snapshot en este momento.
Translated from Spanish would be:
Running dart2js...
Didn't expect \dart-sdk\bin\dart2js.snapshot at this moment.
I'm running Windows 7, 64 bits.
Dart Editor version 0.2.6_r15355
Dart SDK version 0.2.6.0_r15355
Note: I used to be able to run dart2js successfully, now I get that error.
Update: I've tried it with a nightly build and it worked well.
Per the comments above, try switching to a nightly build. Sorry for the bug. I assume it was a bug and that it's fixed.