Simplify VM internal representation of a mixin application clause (MixinAppType

Simplify VM internal representation of a mixin application clause (MixinAppType
object) by removing some redundant field and type arguments.
Tolerate mixins with wrong number of type arguments and update tests.
Note: this is a first cleanup step in preparation of more mixin bug fixes.

This cl was committed as r27363.

[regis, 2013-09-10 23:38:02]

I still need to update the status files for dart2js and friends, since I
modified a language test.

Thanks,
Regis

[regis, 2013-09-10 23:56:20]

On 2013/09/10 23:38:02, regis wrote:
> I still need to update the status files for dart2js and friends, since I
> modified a language test.
> 
> Thanks,
> Regis

Language tests status files are now updated.

[hausner, 2013-09-11 00:05:23]

Lgtm. Nice cleanup.