Remove assert breaking pub after mixin fix.

Clarify comment explaining assert in Function::SetParameterTypeAt.

R=zra@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41447

[regis, 2014-10-30 18:24:07]

regis@google.com changed reviewers:
+ zra@google.com

[regis, 2014-10-30 18:24:08]

This problematic assert was added recently and I do not quite understand its
purpose.

[zra, 2014-10-30 18:32:17]

See these changes from Ivan:

https://codereview.chromium.org/664593002/
https://codereview.chromium.org/652743007/

The rationale for the assert is that when the kind is message extractor, then
the parameters should have already been set.

[regis, 2014-10-30 18:39:09]

On 2014/10/30 18:32:17, zra wrote:
> See these changes from Ivan:
> 
> https://codereview.chromium.org/664593002/
> https://codereview.chromium.org/652743007/
> 
> The rationale for the assert is that when the kind is message extractor, then
> the parameters should have already been set.

Thanks!
But this is not strictly true, since a method can be cloned (including its
result and parameter types) after it has already been finalized. My last cl
(https://codereview.chromium.org/646103005/) uses SetParameterTypeAt to adjust
the parameter types while cloning.

[regis, 2014-10-31 20:02:14]

PTAL

After talking to Ivan, I understand why this assert is required. I updated the
comment instead of removing the assert.

[zra, 2014-10-31 20:03:03]

lgtm

[regis, 2014-10-31 20:14:59]

Committed patchset #2 (id:20001) manually as r41447 (presubmit successful).