Make saving and restoring of the context around closure calls explicit.

Make saving and restoring of the context around closure calls explicit.

This is a first step to refactor the codegen-templates to be reused
between optimizing and non-optimizing backend.

The context is now explicitly saved in a temporary local variable and
not passed as an argument to the ClosureCall instruction anymore.
Committed: https://code.google.com/p/dart/source/detail?r=7811

[srdjan, 2012-05-21 17:43:47]

LGTM