Remove non-useful duplicated code from the flow graph builder.

Remove non-useful duplicated code from the flow graph builder.

For most AST nodes, there is a core of the translation that is shared
for all contexts and only the handling of the final computation
differs.  Make this last part virtual and the visit functions
non-virtual by default.

We can still override visit functions if we want truly different
translations.

R=srdjan@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=4786

[Kevin Millikin (Google), 2012-02-29 14:25:42]

I think it's clear that the graph builder has so much generality that it's
unwieldy.

[srdjan, 2012-02-29 18:17:44]

LGTM