Update test for assignment to final variables

Update test for assignment to final variables

The latest spec (0.51) says:

Attempting to assign to a final variable anywhere except in its
declaration or in a constructor header will cause a runtime error to
be thrown as discussed below. The assignment will also give rise to a
static warning.

R=ahe@google.com
BUG=7062, 11591
Committed: https://code.google.com/p/dart/source/detail?r=26519

[Søren Gjesse, 2013-08-22 12:50:31]

Updated the CL with removal of final_var_negative_test.dart as it does not test
anything not already tested by final_variable_assignment_test.dart.

[ahe, 2013-08-22 13:03:55]

lgtm

AWESOME!

[Søren Gjesse, 2013-08-22 13:15:32]

Committed patchset #2 manually as r26519 (presubmit successful).