Reapply "Don't try to box immutable objects."

Reapply "Don't try to box immutable objects."

This reapplies (a fixed version of) commit 6631 which was reverted in commit 6632.

Fix tests (types were wrong).

Committed: https://code.google.com/p/dart/source/detail?r=6639

[floitsch, 2012-04-17 13:35:20]

Only changed the tests (function 'fun' does not have 'void' return type
anymore).

[ngeoffray, 2012-04-17 13:38:02]

LGTM :-)