Improve type optimization reusing the type argument vector of the instantiator

Improve type optimization reusing the type argument vector of the instantiator
of generic objects:
Do not require anymore that the vector be of the same length. A longer vector
with a compatible prefix is acceptable. This saves a class id check and length
check.
We still require that the uninstantiated type argument vector be the identity
vector, i.e. consisting only of type parameters with consecutive indices
starting at zero. We will relax this requirement in a later change.

Committed: https://code.google.com/p/dart/source/detail?r=22062

[srdjan, 2013-04-25 23:05:31]

LGTM, I think ;-).

[regis, 2013-04-26 00:04:19]

Committed patchset #1 manually as r22062 (presubmit successful).