Try removing explicit use of dynamic

Try removing explicit use of dynamic
Committed: https://code.google.com/p/dart/source/detail?r=13486

[Alan Knight, 2012-10-10 17:16:10]

TBR.

Still getting errors, the messages appear to be related to types, let's try
removing the possibly tricky explicit use of dynamic as a return type. If that
doesn't work, I'll just revert the whole thing.

[Emily Fortuna, 2012-10-10 17:45:19]

lgtm