Take DynamicSelf as a keyword, but parse it as a type-identifier.
Teach function declaration checking to sniff out and validate
DynamicSelf early, with appropriate QoI for references to DynamicSelf
that appear in other places.
As a temporary hack, DynamicSelf resolves to an alias for 'Self' in a
protocol or the enclosing nominal type.
Swift SVN r12708
