Move temp_index from the graph visitors to the graph builder.

Move temp_index from the graph visitors to the graph builder.

This is the only bit of state that besides the current instruction
that is threaded through the visitors.

R=fschneider@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29801

[Kevin Millikin (Google), 2013-11-04 09:29:52]

I'm not entirely thrilled with this change.  The original intent was to keep
state only in the individual visitors and out of the FlowGraphBuilder, so that
the graph construction would not be unnecessarily dependent on the order of
visiting the AST.

At this point, there is a lot of mutable state in the flow graph builder and
it's quite tangled, so there is no real benefit to keeping this single piece
separate.

[Florian Schneider, 2013-11-04 10:20:46]

LGTM. It does make sense to have the state in one place. #args pushed is already
kept in the builder, and not in each visitor. So I think it's ok to move
temp_index as well.

[Kevin Millikin (Google), 2013-11-04 11:48:51]

Committed patchset #3 manually as r29801 (presubmit successful).