Improve and fix constant propagation for type arguments.

Improve and fix constant propagation for type arguments.

Corretly match the constant propagator's analysis code for
InstantiateTypeArguments with the code generator template for it and handle
the case of NULL-type arguments.  This improves code like

new List()

in checked mode. The old code would only improve code with type arguments like

new List<type>();
Committed: https://code.google.com/p/dart/source/detail?r=15695

[srdjan, 2012-12-04 15:51:52]

LGTM and thanks