Issue 6971. Fixes for using built-in identifier as import prefix.

Issue 6971. Fixes for using built-in identifier as import prefix.
http://code.google.com/p/dart/issues/detail?id=6971

Only first part of issue.
I don't agree that code like this is valid:
   abstract<dynamic> v;

I think "abstract" here is still type annotation, so error should be reported.


R=brianwilkerson@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=16086

[Brian Wilkerson, 2012-12-12 22:50:10]

LGTM

Given that the spec states that "It is a compile-time error to use a built-in
identifier other than dynamic as a type annotation.", I would agree that
"abstract<dynamic> v;" is not valid.