Better error message for const loop variables

Better error message for const loop variables

issue 5629

The new error message is:


'file:/n.dart': Error: line 4 pos 8: Loop variable cannot be 'const'
  for (const i in [1,2,3]) {
       ^
Committed: https://code.google.com/p/dart/source/detail?r=13255

[regis, 2012-10-04 19:30:05]

LGTM

It would be good to file a co19 issue for the failing test and change the
referenced issue from 5629 to the new one.

[hausner, 2012-10-04 19:43:56]

Done. Thanks for the review.