Reapply r27640: "Emit compile-time error on duplicate parameter names in typedefs."

Reapply r27640: "Emit compile-time error on duplicate parameter names in typedefs."

R=johnniwinther@google.com, ngeoffray@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27693

[karlklose, 2013-09-20 08:52:48]

The original CL in patch set 1 has been reviewed by Nicolas.

Patch set 2 fixes a problem in dart2dart: since we now create elements for the
typedef parameters, dart2dart tries to create placeholders, but it does not have
a context for them.

Perhaps we should create these placeholders, but for now I'll just leave it as
it was before.

[Johnni Winther, 2013-09-20 08:59:29]

lgtm

[karlklose, 2013-09-20 09:55:55]

Forgot a place and the status file.

[karlklose, 2013-09-20 09:59:21]

Committed patchset #4 manually as r27693 (presubmit successful).